So, my car got a 300,000 mile refresh. It's a M47. I changed as much as I could including, IAC, FPR, Plugs etc. I cleaned everything else and checked for vacuum leaks. Of course now I have high idle and surging sometimes. I adjusted the idle with the screw as low as it will go. TPS makes a click on the slightest opening. But if I blip the throttle, at idle it will settle down below 1000 RPM. Sometimes it idles correctly on it's own. Every now and then it will backfire when shifting.. If I put the AC on, (I am near Houston) it knocks the RPM's down a good bit. What should I try and why does blipping the throttle help?

https://www.brickboard.com/RWD/volvo/1684595/220/240/260/280/solved_surginghigh_idle_bogging_backfiring_update.html
--
Art Benstein near Baltimore

Blipping the throttle does the same thing as whacking the side of a TV did in the last century. The TPS should not be clicking at the hairy edge of throttle movement, but needs just a tad of free play to be sure the contacts make reliably. The click doesn't actually correspond to the electrical function as it would seem.

Along with your refreshed IAC valve you might want to find a new TPS, but try just backing it off a hair from that hairy edge.
--
Art Benstein near Baltimore

“Expertise in one field does not carry over into other fields. But experts often think so. The narrower their field of knowledge the more likely they are to think so.” – Robert A. Heinlein

Thank you for the response. It is a new IAC. It must be the TPS or linkage or idle screw.

Hi,

That’s correct!

In most cases something is not letting the closing down of the throttle plate happen.
It’s the only way the engine should be getting extra controlled air besides an air leak else where that is uncontrolled. The vacuum lines or hoses are the preferred first choices for being suspects.
The next place is the stop screw under throttle shaft lever, that’s opposite the TPS gets messed with or the TPS is holding the throttle open.

Make sure there is some slack in the throttle cable going over pulley diameter holding the linkage rod that Dave and Art talks about.
The tabs, on the pulley, must not hit any stops prematurely upon closing.
A little space is required away from them and this is where the linkage rod cannot be too short.
If, Made too long it doesn’t seem to matter very much there will be just a larger gap.

Everyone likes a throttle to close, when it needs to be!
Similar to having the brakes work too!
(-:)

Phil

The TPS might be too close to a hair trigger for sure. But the weird thing is that if I force the throttle wheel down, it idles fine. Almost like the spring return is off or something. I will mess with it and see.

That's the TPS adjustment.
--
Art Benstein near Baltimore

He often broke into song because he couldn't find the key.

Art,

So even with the idle stop screw OFF of the TB, linkage disconnected and the TPS engaged it is still idling high at about 1200-1500. It still surges some too (its an M47) so I am at idle a lot.

If I can turn it by hand (further closing the plate) and make it idle down to maybe 400 rpm (barely running) wouldn't that suggest that the spring isn't closing the plate all of the way? I think that is why blipping the throttle can lower it sometimes. What can I do? Should I screw with the spring? I need to replace the shaft seal as there is a vacuum leak anyway on the spring side. All of the standard procedures have been tried.

The surging is a symptom of the TPS not closing. The air for the high idle could be getting supplied by the IAC. You tried backing off the TPS adjustment, I take it, but it hasn't helped. As I said before, Along with your refreshed IAC valve you might want to find a new TPS, but try just backing it off a hair from that hairy edge.

As Dave suggests, you want to test the function of the TPS. Problem is a meter, or the OBD mode 2 test is great at identifying a solid, hard fault, but neither is good for checking the kind of intermittents you used to cure by whacking the side of the TV set, or in this case, blipping the throttle. You'd need an LED test lamp or an oscilloscope to see the kind of flakiness I'm betting you've got.
--
Art Benstein near Baltimore

"By the way if, you're wondering where the Bluetooth name originally came from, it named after a Danish Viking and King, Harald Blåtand (translated as Bluetooth in English), who lived in the latter part of the 10th century. Harald Blåtand united and controlled Denmark and Norway (hence the inspiration on the name: uniting devices through Bluetooth). He got his name from his very dark hair which was unusual for Vikings, Blåtand means dark complexion. However a more popular, (but less likely reason), was that Old Harald had a inclination towards eating Blueberries , so much so his teeth became stained with the colour, leaving Harald with a rather unique set of molars. And you thought your teeth were bad..." -- Palo Pacific

Well at this point, a new TPS is a small price to pay in relation to the frustration level! But it runs awesome off idle. All that other work really paid off. Much more power and responsiveness. It is funny how short of a component dying completely they can lull you into a poor running car without ever realizing it. But changing so many parts at once doesn't help identify what was the offending piece.

But I did a bunch... Plugs, cap and rotor, new IAC, new FPR, new gaskets, new Flame Trap components, Air filter, timing belt, accessory belts, cleaned TB and my intake manifold, all new vacuum lines, new fuel filter and new fuel pump, all fluids and probably more that I am not thinking of. That's why this is even more frustrating!

Just in case I missed something .. don't I recall the 1990 240's built with a manual transmission were outfitted with LH3.1 fuel systems? Or was that started in '91? Anyway, how many pins on your AMM's electrical connector. If just 4, your TPS is a throttle position sensor, not a throttle position switch -- a fancy distinction between a variable resistor and a switch.
--
Art Benstein near Baltimore

Do plumbers who become electricians install outlets upside-down? Hot on the left?

Doesn't the AIC need a base setting by grounding a wire and adjusting the throttle by-pass black knob

This is an LH 2.4 car (Regina is similar). No thumbscrew and no IAC grounding wire. Idle is controlled by the ECU. The target base idle speed is determined by the ECU based on conditions like engine temp (ECT to determine a cold engine) and load (A/C on to prevent the idle dropping too low when the compressor kicks in). The only air bypass around the throttle plate is the IAC valve (whereas LH 2.2 has an additional air bypass using that thumbscrew or in some engines a large set screw). The throttle plate stop screw is the only adjustment, set to keep the throttle plate open just enough so that the IAC will be operating more or less mid-range so that the ECU can then adjust idle or up or down from there as needed. Although adjustment of the stop screw can be done with the IAC air hose pinched off much like LH 2.2 setting it to a "base idle" rpm spec, with LH 2.4 a better and much simpler method that does not involve pinching hoses, a running warm engine and an accurate tach is to adjust the stop screw using what's known as the 1/4 turn method, advancing the screw 1/4 turn past where the screw is touching the stop and about to start opening the plate. LH 2.2 throttle plates can be similarly adjusted using the 1/4 turn method followed by using the thumbscrew to set the base idle to spec with the IAC forced to stay in a closed position by grounding that wire (alternatively you could just pinch off the hose). The IAC valves in the two systems are slightly different. LH 2.2 has a spring loaded piston valve while LH 2.4 is floating.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

"LH 2.2 has a spring loaded piston valve while LH 2.4 is floating."

Are you sure this is correct? Shouldn't it be the other way around?
The 2-wire IAC on my LH Jet 2.4 has a pretty strong spring in it.

It's LH 2.4. I know there is one pin absent in the clip. Not sure if there are 4 or 5. I seem to think it is a 6 pin female connector with only 5 connections. One is not used.

Yes, 6 positions on the connector, only 5 used. LH2.4 doesn't need the one pin that used to convey the LH2.0 and LH2.2 CO adjustment to the ECU. LH3.1 has a 4-position connector; all pins used.

Thanks!
--
Art Benstein near Baltimore

Down in the depths of our souls, we all yearn for a job where rubber stamps are used.

Art, I am about to blow a gasket (no pun intended). So after all of the other stuff that I have done, I installed a new AMM, TPS and Temp Sensor for the ECU. Same exact issues! Does OK when cold at idle then high idle, occasional idle fluctuations, bogging when accelerating and some backfiring. Like I said, I can blip the throttle and stop the fluctuations but not the high idle.

LIke I said earlier, I have installed basically everything new including Cap and rotor, plugs, intake and TB gaskets, new accordion hose, new IAC, new FPR, New fuel filter and main pump. I have smoke tested for vacuum leaks and there is a tiny one out of the throttle body shaft seal that I put a new seal in and it knocked it down a lot.

Here are my issues that I have not addressed:

1. New Spark Plug wires (coming tomorrow). No evidence of arcing that I can see.
2. I have a vacuum gauge in my cluster and it stays at 20 and then when accelerating it sometimes goes down to goes down to under 5. I believe there is a leak in the vent controls evidenced by the air moving from the floor and dash vents on acceleration. The vacuum gauge is teed off of the AC vacuum line. The vacuum does seem to get erratic sometimes. I cannot find any obvious leaks and nothing shows up on smoke test.

Can the vent control cause that big of a vacuum leak to affect drivability?

Is it possible that my Computer brain is messing up? Years ago I put in the better one (can't remember the last three numbers).

Hi,

If you think it’s the vent controls causing the issues then find that hose going into the firewall and pinch it off completely.
You don’t need to operate those vents during troubleshooting.

I assume you have checked or adjusted the throttle plate to its most closed position.
It getting more air than it should at the wrong times from one symptom.

Lean mixtures cause misfires. Back firings are considered done or perceived in two ways. Back up through the intake manifold or out the exhaust from excessive fuel dwelling in the exhaust pipe manifold.
Carburetors dump fuel in the intakes to cause those. If it goes through the engine not burned then the exhaust sounds off!
Fuel injectors don’t mess up that way! Computers don’t mess up that way but it does work off information received. You did change the TPS and it has a limited function overall.

The backfiring is either the timing jumping around or a serious air leak for the leanness or lack of power.

What is strange here is the fact that blipping the throttle settles things down momentarily.
Sounds crazy I’d admit!

If I didn’t know better I’m starting to wonder if it’s a intake manifold gasket leak, but you did a smoke test?
Even then, that would not affect the acceleration moments with some backfiring and passing that smoke testing? Unless it is not a fool proof diagnostic procedure. I have no experience with them!

This puts me to thinking about the CPS component as it does effect timing. Timing effects power and idle speeds! Have you changed that thing? I ask, because I have not read all the posts!

I don’t how many different ways those Sensors can go bad.
This problem is having many variables and it sure would make be me blow a “head” gasket too!

I feel for you!

Phil

Thanks for the reply. Yes, I did the timing belt about 10K miles ago. The car ran the same before and after the timing belt change. It also runs perfect every now and then. I. am wondering about the ground issue that Art suggested with the TPS (at least for the idle issues). I need to figure out where that ground wire from the TPS goes to and see where it eventually grounds out. The backfiring and stumbling shouldn't be because of that.
I have seemingly chased down all of the fuel issues. It has to be spark or air. Like I said before every now and then the vacuum gauge (in my car that I added) does show really low vacuum sometimes on acceleration (down to zero) and sometimes it idles at 14-15 instead of 20 then it goes away. It is driving me nuts.

When i first start it and it isn't really hot outside it idles at the right RPM's just rough. But it drives great without hesitation. Its an M47 so it goes to idle a lot when I am coasting to lights etc.

The only code that I have had (one time) since I changed all the stuff is 1-3-3. But if I unplug and idle it and plug it back in it makes a big change. And it is a new TPS. I am wondering if it is the harness or something in the ECU, like the ground Art references. But I can't find any diagram on it.

The other problem is that my old trusted Volvo mechanic died. He was awesome, he was from Sweden and worked in the plant for Volvo and came to the Houston area in the mid 70's. is name was Nils Nillson. Houston area Volvo people knew him. He knew those cars better than anyone. Now I have kind of been on my own the last 10 years. I have managed to keep in going just fine until now. I am at wit's end.

Hi,

I wasn't talking about the timing belt, it was the Crank Position Sensor located behind the head above the transmission.

Backfiring and stumbling and loss of smooth vacuum reading points to the timing jumping around.
You are observing the drops in vacuum readings and it's expected to happen the more you open the throttle plate.
But it not returning to a high state, repetitively, is not normal.
I wish I could see the vacuum needle and how the fluctuations are, if any?
The gauge should be right on the manifold, not on a restricted AC line for accurate readings.
No check valves and alike included.

Ignition Timing or a malfunction of the throttle plate/idling system combination is causing an uncontrolled air supply.
It might be possible to use a timing light and watch for the mark to change abruptly while moving the throttle speed. The advancing portion should be steady to move and repetitive to each idle moment.

In the old days we blame it on a worn cam shaft in the distributor, sticking vacuum diaphragm and similar ignition issues with the contacts of the point system. Dwell use to be the big deal in those days! Breaker less ignition was a large leap for ignition systems.
On the 1989-on the distributor function is nothing more than an "intermediary item" in the spark distribution system.
The CPS is much more important!
Only, out done by the timing belt itself!

As far as ground yes it's a possibility on a very abused car!
Has the car had water leaks in the areas of the EZK system?
I just don't get that from you in these posts!

As for the ECU, well that's a tree I don't go barking around unless I'm absolutely sure there's game up in the tree!
You say it runs great, so this dog, stays on the porch with that statement!
:-)

Phil

"I am wondering if it is the harness or something in the ECU, like the ground Art references. But I can't find any diagram on it."

I always thought that the TPS would simply earth somewhere on the chassis.
However, I had a look in the schematics, and it seems that it grounds somewhere specific.

The image below is from the 85 schematic on the B230F. I took it from that one for clarity, but this bit is the same in the 86-93 schematics.

The interesting bits are the 1 and 2 in a square. The legend says that 1 is signal earth and 2 is output stage earth.



104 is the ICU;
218 is the TPS;
221 is the ECT;
222 is the idle speed adjustment.

"The interesting bits are the 1 and 2 in a square. "

Yes, interesting to me, because those two are the grounds at the fuel rail mounting tabs. My understanding has always been the one closest to the firewall is the "power" ground; that is a ground return for loads such as the fuel injection computer, ignition computer, power stage, etc. High current loads. The signal ground on the other tab, closer to the front of the car is for "signal" returns, separate so the fluctuations in loads don't affect signal voltages as much as if they shared the return path.

In the '91 WDM page 10 the ground connections are listed just the same as in your excerpt, and instead of the TPS returning with signal grounds, it is spliced in with the power grounds at 2, near the firewall. The splices are hidden in the vinyl sheathing, and those splices have been cited by others as a problem.

Anyhow the black wire from the TPS plug winds up in one of those splices -- the one finally connected at the fuel rail ground nearest the firewall. Surely if that screw were loose, the ground reference would fluctuate with mechanical vibration and load changes.

Here's a picture of one of those splices with the black vinyl sheathing cut back for inspection.


--
Art Benstein near Baltimore

"It's been one of those days. Morning DJ moved to the afternoon. Busted a manifold flange stud. Filter slipped from my fingers into the bucket."

Wow, thank you. I was wondering what would happen if I just bypass the whole IAC system by disconnecting the vacuum lines and running a hose from the accordion hose to the throttle body, unplugging the TPS and then just adjusting the idle when the engine is warm. I live near Houston and it doesn't get that cold here, meaning the car will reach operating temperature fairly quickly. The question is, do I need to jump the plug to the TPS and tell the computer that it is off-idle or WOT? Is that a crazy idea?

I am going nuts. If if had consistent symptoms that would be great, but it never does.

I don't actually know what experimenting with the TPS that way does from experience. I don't expect it to be a good idea, otherwise we'd seen "fixes" disabling them long ago (like removing an EGR). I'm not aware of any, but perhaps someone (anyone?) else on this forum has, maybe as a temporary fix?

The TPS is instrumental in telling the ECU when to control the idle. WOT will probably only set the ECU and EZK to a state that prevents knocking. And the rest of the pedal travel in between is left to the driver.

I know from measurements I took that the IAC will also be controlled when the vehicle is moving without any throttle applied (e.g. engine braking downhill) to prevent inlet manifold vacuum from becoming too high. Without a functioning TPS, this wouldn't work.

So I removed the IAC hoses and plugged the holes, unplugged the TPC. It is idling perfect at around 800rpm. I didn't drive it but when I increased the throttle, it revved and returned to idle normally. However it is still hiccuping when accelerating and then it settles at the higher RPM. Before I did that, I played with the TPC settings and tried all extremes and in between. It definitely affected the idle as it should, but after accelerating at a normal pace it would return to a high idle where there wasn't one before or even go down to 300rpm and die. It must be a ground. I need to try and find the wire Art said and see if it is grounding out erratically or something.

I did move my vacuum gauge location from the ac vent port on the manifold to "y" it off of the PCV nipple on the manifold. I do have a leak in my vent system for sure. I can feel the air on my feet when the vacuum drops. Erratic, but sometimes I can hear it hissing. But I plugged it and it doesn't seem to affect drivability.
I romved the kick panel and it seems fine into the vacuum resevoir, the pods are moving that I can see and hear after removing the glove box. I am wondering if it is the recirculation one.

Interestingly plugging the holes where the IAC hoses went did seem to help. I wonder if the hoses were old and hard and not sealing well.

So the IAC is causing the high idle and hunting issue for sure. But I still have the stumbling and occasional backfiring. So the crank sensor could be screwing up something that would cause this?

[NB I've been heavily editing this post for typos and accuracy so don't go by your email notification]
Hmmm, if the engine is idling nice and normal and you then disconnect the IAC valve hoses and plug the hose nipples on the throttle body then idle should not remain at 800 rpm, it should drop to around 500 rpm or below, possibly even stall out. Something very basic is wrong. If it was a ground wire problem I would expect your symptoms to be much more intermittent than you've been describing where you can create the high idle issue at will by racing the engine and slowly letting the throttle close on its own. Before we start, when checking base idle, please remember to have the A/C off and the heater not in defrost.

a) If the behaviour is as you've been saying with the engine capable of running half smoothly at a proper idle of around 850-950 rpm with a fully warmed engine and as well running half decently with the throttle part way open say up around 1500-1700 rpm and plugging the throttle body air bypass does not drop the idle way below 800 then I can only conclude your base idle is still incorrectly set. You likely have other issues as well affecting hesitation when you accelerate away from idle and as well idle still dropping after you blip the throttle or manually force it closed, but you have to start with the basics as I've tried to previously suggest. Please go back and carefully review each of my earlier postings regarding throttle body adjustment. Please follow the link to my post in another thread that details throttle body and linkage adjustment procedures. You will need feeler gauges to do this, no tach or engine running is involved. Fully loosen the TPS switch and adjust the idle stop screw using the 1/4 turn method. Every step is important, right back to making sure the throttle cable from the pedal has slack and can retract easily on its own -this could easily be the OPs problem. If you've got a meter, please check the TPS switch for continuity between pins 1-2 and then 2-3 at the closed and WOT positions -not to worry if you don't have a meter, we'll verify TPS functionality at the next step.

b) Now as for the idle remaining too high after dropping after you blip the throttle and if everything is as you've been describing then there are really only two possibilities:
i) The ECU somehow doesn't know it's supposed to be maintaining idle. Please do as I previously suggested and use the OBD connector to run DTM 2 System Sensor Signal Test. It's described in the FAQ. If the blinking LED won't go out when you do the first step manually opening the throttle and can't get the 3-3-4 code then you know it can't see the TPS switch opening and closing for whatever reason, be it mechanical or electrical and you can then pursue that issue. In order to try catching it misbehaving, while the idle is running too high then just shut the engine off before doing the OBD test.
OR
ii) The engine can't properly control idle. That puts my thinking right back to my very original post about a sticky IAC. You said you recently replaced the IAC valve. Did you replace it with a new or used one? With the engine fully warmed and the idle running too high, tap solidly on the IAC body. Without touching anything else if the idle now drops or otherwise changes then you know it's sticky. Next, go back to the FAQ and perform OBD DTM 3 Component Activation. It will keep cycling through the various component tests until you stop it. One such test is fully opening and closing the IAC piston. Can you feel and/or hear the IAC valve going tic-toc? If not then the IAC is either faulty or the ECU can't talk to it.

Please report back with how you make out and we'll help you take it from there. Once you've got idle control behaving properly you can start worrying about getting it to not stumble so much when quickly opening the throttle.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Further to my previous post today, could a bad knock sensor be causing the bogging and backfiring?

Re. bad knock sensor. I wouldn't at all suspect the knock sensor being so reponsible for such hesitation. If the knock sensor fails completely then a DTC will be set. If it's misbehaving then you may have timing problems more likely to be noticed by sluggish response and poor gas mileage, but not the kind of hesitation you describe, especially if the engine accelerates relatively normally once fully warmed. I always kind of get a kick out of people suspecting knock sensors problems when faced with a mystery. I was guilty of that myself early on with my LH systems, to the point of worrying that under torquing the fastener made a significant difference. Faulty knock sensors are now way down on my list of suspects for basic running issues such as yours. See the Engine Sensor section of the 700/900 FAQ here if you want a bit more info.

Getting back to your problems, I know you're eager to get on with dealing with cold hesitation, but you should perhaps get that idle problem under control first. You're almost there. You are now focussed on the IAC as the main culprit behind your idle control issues, which right from the start was my primary suspect. Can you confirm you've now re-adjusted the TB/TPS to spec and verified TPS operation with OBD DTM 2? If both the IACs you've tried aren't able to control idle then that certainly points the finger at an electrical connectivity issue.

I will ask you yet again, did you try doing OBD Diagnostic Test Mode 3: Injection System Component Activation Test as I suggested? -it's fully described in the 700/900 FAQ. And neither of the IAC valves you have went tic-toc during that test? But you could hear/sense the injectors ticking (the preceding test in the sequence) which confirms you were actually running DTM 3? Only if you've done all that should you suspect the IAC connector wiring through to the ECU. You would then inspect the IAC connector carefully for perfectly intact pins and peel the rubber boot back to confirm the integrity of the wiring. Subsequent to that would be using a meter to verify continuity between the connector pins and the ECU connector. I've been unable to find an LH 3.1 pinout that shows which pins the IAC are on (the schematics I found show everything except the IAC, go figure). If you get to that stage then you're going to either need more help here from someone with an LH 3.1 manual or have to carefully inspect wire colors at the ECU to identify which pins. Also, if you do get to that stage then be sure to probe with the meter at the side of the ECU connector, not into the connector pins themselves as they are too easily bent and damaged.

Once you've confirmed all that we can get back to addressing your cold hesitation issues.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

You are awesome to respond. I have 2.4. Does that change anything?

LH2.4, right, I must have been thinking of another thread when I saw a 90+ 240 w/M47. That means we’ll have easier access to wiring diagrams and most of us aren’t as familiar with the LH3.1 differences like the AMM/MAF and TPS. That doesn’t change any of my thinking on your problems. I’m still curious about your OBD testing.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

I have tested 2 and 6 and got no codes. I was unfamiliar with 3. I will look that up.

You can do more than check for DTCs (diagnostic test codes) on port 2 & 6 at the OBD connector (which typically, but not always, also light the Check Engine indicator).

The OBD connector can also be used to run the diagnostic procedures I've been suggesting. The procedures are summarized here in the 700/900 FAQ
https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm#FuelInjectionandIgnitionDiagnosticCodesi
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Ok, I went through all of the diagnostics. Everything checked out except it said that it had no signal from RPM sensor. However on the function test, I heard the injectors and the IAC back and forth and that was it. I never heard the fuel pump, although it is clearly pumping. Could the bad RPM sensor cause the bogging and backfiring? I took it off and cleaned it, the wires were perfect, no fraying or degradation of the insulation.

Idle is actually better and more stable after I plugged the IAC back in.

Great, doing and passing those tests removes a number of possibilities.

Don't worry about failing the RPM sensor test (CPS) in DTM 2 for now. For LH 2.4 it is incorrectly documented in the FAQ. The Volvo procedure does not say to disconnect the coil wire and the code you're looking for is not 1-4-1. It should read "With the LED flashing in DTM 2, try starting the engine. If the LED turns off then flashes the 3-3-1 code then the RPM signal is correct. If the engine won't start, run the starter motor until the the diode turns off. If you now get the 3-3-1 code, the RPM signal is correct, otherwise continue diagnosis. If the diode continues to flash rapidly then the ignition system must be checked, the fault could well be the RPM sensor, but possibly could be elsewhere, continue diagnosis."

That's interesting about idle improving after running DTM 3. When I've had that happen it's almost always a sign of a sticky or worn IAC. DTM 3 rocks the IAC piston full travel which may loosen grunge (and in the case of a badly worn LH 2.2 IAC valve may slightly smooth out the burr at the end of the contact wear grooves), just enough to have it run better for awhile. To prolongue the life of a sticky IAC I once had, I used to let DTM cycle through a great many times and the idle would stayd more stable for a couple of months before acting up again.

My next questions to you:

1) Did you test both of your IACs with DTM 3 and they both did the tic-toc?

2) Is the "new" IAC you're using brand new or used? Genuine Volvo or an aftermarket rebuild?

3) Do you have a Bosch or VDO TPS switch?

4) Describe for me how far you had to slowly open the throttle during the DTM 2 test before it clicked, the LED stopped flashing and you got the 3-3-4 code. Did you barely have to turn it before it clicked? Please repeat the test with a double folded scrap of standard bond paper inserted as a spacer at the idle stop screw (presuming you don't have the requisite 0.15 mm feeler gauge) -it should still click and pass the DTM 2 test. Now fold it 5 sheets deep and try again (an approx .45 mm feeler gauge) -it should now not click or pass the test (if it does click try a 6 layer spacer just to be sure before declaring it over-adjusted).

--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Well dang, I ordered a new crank sensor. So here are the answers to the questions:

My next questions to you:

1) Did you test both of your IACs with DTM 3 and they both did the tic-toc? No, just the new one. It worked perfectly. I mean it cycled fully.

2) Is the "new" IAC you're using brand new or used? Genuine Volvo or an aftermarket rebuild? It is a new one. Got it from FCP. Bosch.

3) Do you have a Bosch or VDO TPS switch? I believe Bosch. It is new as well.

4) Describe for me how far you had to slowly open the throttle during the DTM 2 test before it clicked, the LED stopped flashing and you got the 3-3-4 code. Did you barely have to turn it before it clicked? Please repeat the test with a double folded scrap of standard bond paper inserted as a spacer at the idle stop screw (presuming you don't have the requisite 0.15 mm feeler gauge) -it should still click and pass the DTM 2 test. Now fold it 5 sheets deep and try again (an approx .45 mm feeler gauge) -it should now not click or pass the test (if it does click try a 6 layer spacer just to be sure before declaring it over-adjusted).

It is adjusted with a slight space before clicking. I used a feeler gauge. Still isn't much though. But there is travel, it isn't hair triggered.

It is honestly idling well, it sticks every now and then at high RPM. I wonder if all the plugging and unplugging helped the connection or something. Remember it ws inconsistent idling before. Sometimes I think the IAC would open or not and at the wrong times. The biggest problem is the bogging and backfiring. That happened with the IAC plugged in or unplugged. I checked timing and was off a tooth on the cam gear. Corrected that and it MAY have made a difference, but not much. Bogging isn't as bad but still backfires when getting on it in 3,4 and 5th gear. Then it catches up and it runs fine at speed. One thing that is irritating is my new Bosch fuel pump is noisy. It is whiney, sounds like a distant train. I put on a new filter as well. I read about it and apparently some pumps just whine. Makes no difference if the tank is full or not.

Okay, let's ignore the idle system for now. You've answered my basic questions. Still something wrong if it randomly gets stuck at a higher idle and monkeying with the throttle makes an immediate difference. Maybe try selectively tapping various areas to see if you can isolate it a bit better.

Sorry about the RPM sensor. I'd forgotten the FAQ was wrong until you mentioned failing that test. I replied as fast as I could after reading it and being able to check my manual for the proper LH 2.4 procedure. Unfortunately no one is maintaining the FAQ at the moment to correct it. The same or older versions of that FAQ can be found on multiple other Volvo sites, but the brickboard was considered home base.

Following your train of thought on the fuel pump as being behind your cold stumbling and hesitation, your symptoms are somewhat consistent with fuel starvation under acceleration when demand for fuel is at its highest and with a cold engine combustion is less efficient. Our fuel pumps generally aren't as noisy as you describe, especially proper Bosch ones, unless they are badly worn or there is a restriction in the fuel supply, especially at the tank end, but also at the fuel rail. If there was any possibility of the in-tank pre-pump being mis-wired and running in reverse it would likely stumble more than this, but a new main pump may be able to overcome that. Any chance there?

Ideally you would check pressure at the fuel rail, but few of us shadetree types have the right gauge setup. A tire pressure gauge can be used in a pinch if you've got a schraeder valve, but that may not be accurate enough for a subtle problem. Maybe go into the FAQ and see if you can diagnose a problem at the fuel pressure regulator.

I don't like to get ahead of your testing and thoughts, but in the back of your mind please keep possible ignition issues in mind. I had a poorly gripping spark plug wire on #4 cyl that caused severe stumbling moving away from a relatively decent idle when the engine was started cold and this would completely go away after just a few minutes of warmup. Crimping the boot with pliers easily fixed that. When a mystery has symptoms changing with operating temperature, I'm always keeping poor electrical connections in the back of my mind.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

I appreciate your help, I really do. I installed new cap, rotor, leads and plugs. My mechanic friend who has not seen the car, said that it sure sounds like a timing issue to him. I think I have that sorted. Seeing that it runs the same on full tank or not would lead me to believe that the in tank is working. I guess I can pull all of that and maybe see if the sock is clogged. But the backfiring sure seems like excess gas to me or the timing. He said he wouldn't think that bad leaking injectors would cause that much of an issue.

Any idea why the new pump is noisy? Do you think a dirty sock could be it?

A noisy and relatively new pump and symptoms consistent with slight fuel starvation certainly does make you wonder about the screen sock on the tank pump. Unless something foreign got dumped into the tank, I don't imagine it would be a gummy residue plugging the screen, it would have to be particulate, like flakes of rust. It's not at all common, but in theory could happen. I would expect there'd need to be a fair layer of flaky residue in the anti-slosh bucket at the bottom of the tank to significantly restrict intake through the screen. I'd be more concerned about something having gotten up into the pump intake if the sock got knocked off during installation, it's not that hard to do. I always do a final check with a flashlight to make sure the sock is still on during installation as I did manage to knock one off way back when on my 140.

I know it's a bit of work, but it may be worth lifting the sending unit so you can check the bottom of the tank with a flashlight and as well making sure the sock is still on the end. A careful inspection of the pump is warranted if the sock is off or damaged. Also check the integrity of the pre-pump rubber connector hose and make sure the gas line on the sending unit isn't damaged.

If it's a timing issue, I wouldn't expect it to dramatically improve when the engine is warm, but it's well worth considering.

This discussion is now way down in this thread and others may not be following. I suggest starting a new post to deal with the cold stumbling and hesitation issue now that the blipping throttle idle issue is a bit more under control.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Thank you, I will start a new one for sure. I am going to pull the assembly tonight hopefully and see what is going on.

JUst printed and I am taking it home! I'll report back. Send me a bill!

Control function 3 is activated by inserting the bullit connector into socket #2 en depressing the button three times. Each press should last more than one but less than three seconds.

This results in the operation of the injectors and IAC in succession. I made a video once to show what happens and it can be found here. It's important to play it with sound enabled.
I disconnected the hoses and turned the IAC around for a clear view.

Almost joined the social medium just so I could like them. Makes me want to remake my frozen wiper drive video and choreograph a dance to these.

Bravo!

https://www.youtube.com/channel/UCmsU0s9bx3otmMDnFkn1KXA

--
Art Benstein near Baltimore

Putting the word like in double quotes elicited this response:

403 ERROR
The request could not be satisfied.

So I have concluded that it is an electrical issue (must be) to the IAC. I swapped in my old one and behaves exactly the same. Intermittant high idle, low idle and stalling. I unplugged it adjusted my idle when warm and it is fine. It is rough when the engine is cold of course. My bigger issue is the bogging down and backfiring on acceleration. I have gone through everything. The only thing I can think of is to recheck the timing. I do have a VX cam but I have had it for 20 years. I have not checked the spray pattern of the FI but I can't see that causing all of these issues. I know the IAC is unrelated. At this point, I just want the car to run correct at speed. The acceleration can be babied and it's ok but if I floor it, it bogs. It's a M47 and as we all know, you pretty much have to floor it most of the time in modern traffic. It has new fuel pump and fuel filter. New pump is whining the bit. There are zero vacuum leaks. I have smoke tested it several times. Should I move more toward the 10 mark on the timing indicator and see if that helps. I am at a loss.

I lay my cards confidently down on your ECU not getting the switched ground from the TPS with the throttle closed because you say the idle speed surges until you "blip the throttle." Ordinarily this occurs when the TPS isn't fully closing such as when the click is too close to throttle closed position. But it can also occur if the switch inside the TPS is worn or dirty.

To disprove, disconnect the TPS entirely, and observe the blipping does not stop the idle surging. If it does, my bet is lost.

If this were my car, I'd wire in an LED test lamp at the blue/white wire going to pin 3 of the fuel ECU. I'd use that lamp to prove to me the TPS was grounding every time I took my foot from the accelerator pedal (lamp goes out) before beginning to imagine any other causes.

Of course there are other possibilities in that circuit, but they seem remote to me linking to the blipping action, such as pull-back or damaged pins at the TPS connector, faulty ground, or even a damaged ECU connector pin.

The driveability issues like bogging in accel or backfiring are certainly not idle air control related, in my mind. As for the smoke test flagging the throttle shaft, I don't know of one that doesn't -- just a question of how much, which doesn't communicate well in text.
--
Art Benstein near Baltimore

Two antennas met on a rooftop--, The wedding was O.K., but the reception was great!

Awesome I will try. I can run out here at work and see if unplugging does anything. The other will have to be at home.

So you think the bogging and backfiring is unrelated?

If the throttle plate isn't closing all the way then you need to remove the TPS completely and determine whether the restriction is in the throttle body (throttle plate/shaft/spring) or the TPS switch is internally fouled. I'm suspecting the latter. I know you're itching to be told to put in a stronger return spring, but the odds of it being that are pretty much zero as long as the spring isn't broken, is reasonably clean and not somehow rusted together (spraying it clean with WD-40 is about all the maintenance that spring should ever need).

If neither is obvious then that leak around the throttle shaft bushing may be significant and possibly what's getting in your way. Manually forcing the throttle closed may be forcing the shaft off center enough to seal up the air leak around the shaft on the engine side of the plate (wouldn't matter as much if the leak is on the air supply side of the shaft other than the engine running a bit poorer due to unmetered air getting in). See https://blog.fcpeuro.com/how-to-change-a-rwd-volvo-throttle-shaft-seal

Have you been able to take a meter to the TPS to carefully check out its electrical function? Failing that, have you tried doing the OBD test I mentioned?

Just for jokes, when you've got the TPS off, please measure how far the shaft extends out of the throttle body. If it's somehow extended too far out it can cause the TPS switch to bind and possibly behave as you describe. The spec for adjustment is 22 +/- 0.5 mm, but for the purposes of checking just make sure it's not much more than about 22 mm (7/8").
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

I will take it all apart tonight and see. I need to do that shaft seal anyway. There are no code messages BTW. I have already replaced so much on that car for this 300K freshen up , I might as well buy a new TPS. It is definitely functioning, but maybe it is not letting the shaft return completely. I swear it is like the spring is weak. Something is stopping the plate from fully closing. It isn't much, but enough to bump the idle up an extra 500 rpm's.

The ability to blip the throttle and affect the idle makes me realize it is some thing mechanical for sure, as opposed to electronic. I didn't think that about the mechanical part of the TPS on the shaft maybe being an issue. Good idea.

In an LH 2.4 system, the so called "idle screw" is a bit of a misnomer in that it does not regulate the idle like it would on, e.g., a carbureted engine.
Instead it is used to set a base idle of about 500 rpm upon which the ECU can regulate up to about 750 rpm.

The base idle is from the air passed by the TB, the rest is from air bypassed by the IAC. Because of this, the ECU can only regulate up from the base idle. This means base idle MUST be set first.

The reason this is done this way is to make it possible for the ECU to compensate for extra loads that would drag the idle down, like the AC.
The fact that your switching on the AC drags idle down is a big sypmtom that things are not as they should be.

If you've turned the "idle" screw all the way, I'm pretty sure it's now totally out of adjustment and you really need to use the proper procedure, in the right order, before you do anything else.

Only when performing that procedure doesn't lead to the correct results can you conclude that maybe something is defective.

The procedure can be found at the beginning of this FAQ ("Setting Base Idle Specs and TPS on Bosch LH2.4-Equipped Cars").

Blipping the throttle causes the ECU to force a sticky IAC valve to move and settle into the proper position so your idle can drop as the engine warms up and combustion improves. It's certainly not worth putting in a new IAC, but possibly worth giving the valve area a good cleaning with something like WD-40, flicking the piston back and forth to loosen some of the grunge around the axial bushing, followed by a carb spray rinse (or whatever) the next time you're cleaning the throttle body. Most of us just live with it unless the engine starts to stumble or hesitate at idle, which is also a sign your TB is overdue for a thorough internal cleaning.

As you've been playing with idle adjustment, I will caution you that there are proper ways of adjusting the throttle body, starting with the idle stop screw and then the orientation of the TPS, also making sure that linkage rod adjustment and throttle cable adjustment aren't getting in the way. You can find them in posts here as well as in the 700/900 FAQ (same LH 2.4 as yours). One of the first things I learned with LH systems way back when was that the ECU is in charge of idle speed, not your adjustments, so always keep it adjusted to spec so the ECU can do its job.

If you're having problems with A/C dropping idle, that may well be a further sign of a sticky or worn IAC and even more reason to try cleaning it. Less likely is something wrong with the signal from the A/C switch and A/C delay relay getting to the ECU, but don't worry about any of that until you've got the TB adjusted properly, a clean PCV system, no vacuum leaks, and a smoothly functioning IAC, which are all pre-requisites to having proper idle.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

I put a new IAC on the car so it must be the TPS or idle screw /linkage. What should the car do if I unplug the electrical connection to the TPS? Maybe that's bad.

If it's a new IAC then
a) As Art suggests make sure the TPS is not set on the hairy edge of clicking, either by virtue of the idle stop screw, the TPS orientation, the throttle linkage rod adjustment, even the throttle cable.
b) Check the electrical functionality of the TPS switch (both at idle and WOT). You can do this with a meter, but I prefer to check it using the OBD connector, port 2, DTM 2 (see the FAQ) as it also checks connections and wiring, plus doesn't get my hands dirty.

Make sure the throat of the TB is clean of grunge around the throttle plate and the two little holes, ports and hoses aren't plugged. Also make sure the throttle plate isn't off centre and binding and the shaft isn't extended too far and binding inside the TPS, which should only happen if someone has been inappropriately messing with the plate. One crude way to test that would be with the idle stuck a little high, manually force the throttle fully closed to see if the idle drops like it would if you blipped the throttle. A better way is with the TB removed for cleaning and held to a light to check for possible binding and an even band of light around the throttle plate when fully closed. Of course if the TPS was worn or set on the hairy edge, it would behave similarly, so do attend to that first. You can also do a smoke test or starter fluid spray to check for leakage around a worn throttle shaft bushing.

You asked what happens if the TPS was disconnected. The ECU wouldn't know to control idle and the idle speed can float, anywhere from being unsteady to going high or going low and stumbling, even stalling. By tweaking the idle stop screw you could in theory get the idle close to being correct when the engine is warm, much like you may have done.

The FAQ adjustment procedure for setting LH 2.4 base idle will work, but requires a decent tach and feeler gauges. It is based on a variation of the LH 2.2 adjustment procedure that mechanics were used to.

Most of us can do a perfectly decent job without a tach or feeler gauges just adjusting the idle stop screw using the 1/4 turn method and making sure the TPS click isn't set on the hairy edge. The IAC will now be within suitable range for controlling bypass air allowing the ECU to adequately control idle rpm under varying engine conditions and loads.

For your interest, Volvo actually revised the LH 2.4 adjustment procedures to be more precise in a TSB using just feeler gauges that does not require a tach or pinching the IAC hose and risk splitting it. I recently posted the revised Volvo specifications here:
https://www.brickboard.com/RWD/volvo/1681369/220/240/260/280/throttle_body_adjustment.html
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Thank you for the great response. There was a band of light around the top half of it. I noticed it after I cleaned it. It seems like the spring isn't returning all of the way down or something. I do believe that I have the TPS set too close to a hair trigger response. If I force it closed with my hand on the wheel, the idle drops to normal. It makes me think that maybe the plate isn't closing all of the way. That's even with the idle screw backed completely off where it isn't even touching. So I'll take off the TB and back the screw all the way down and look. If there is a light ring around, then something is off correct?

Hi,

Dave is correct in all the information he is giving you on the throttle plate alignment.

The plate should be centered in the bore equally from the factory.
It is held by screws through the shaft but I cannot recommend that you do anything to move the plate. Those screws are set and “staked” from the back side on most all of these type air control devices!
This is so they will not back off or pieces possibly fall into the engine. Even worse, jam open the plate right when you really need to stop!

The return spring is always applying a twist on the shaft that could cause the shaft to wear slightly offset in its own bore. It’s been my experience that I have never seen a throttle body with enough wear to shift the plates shaft.
So I’m inclined to believe with Dave that there is something else in play here.

Like I have said before the linkage cannot be too long as it will only hold the pulley off it’s own stop and shorten then throw distance to wide open throttle. That’s no big deal as who goes around driving at WOT that often?
But now, too short is a big problem.
Since you can close it, there might be some play or wear in the knuckles allowing the variations or the shaft to be flopping around.
The TPS is not there to stop the pulley so it too has to be considered.

All of this is no big deal as it can be all be compensated for.
The truck is just realizing what you want the whole setup to do and that’s to repeatedly close completely and get all the idle air to go through the IAC valve.
The TPS just needs to give the ECU a heads up that that plate is being moved some to start the “transition zone” to power up the system smoothly.

Once you get it right, you can explain to us what you found, in great detail!
(:-)

Phil

The throttle plate doesn’t seal, you just want to see light passing more or less evenly top, bottom and both sides so you know it isn’t capable of binding and there’s no grunge. The lower edge and throat there are where oil vapour, dirt and varnish accumulate, into the little holes as well. The throttle spring itself is rarely weak and the plate should close easily as long as there’s no excessive dirt build-up. If you can force it closed to get proper idle then either the plate is binding, the TPS is set way too on the hairy edge, or something is preventing its full return, like the linkage rod or throttle cable, or else the TPS switch is internally worn or fouled. I’d disconnect the linkage rod, loosen the TPS switch and back off the idle stop screw then start re-adjustment of everything to spec from square one, beginning with using a meter to electrically check the TPS for consistent good continuity at the click.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

If it's a new IAC then
a) As Art suggests make sure the TPS is not set on the hairy edge of clicking, either by virtue of the idle stop screw, the TPS orientation or the throttle linkage rod adjustment.
b) Check the electrical functionality of the TPS switch (both at idle and WOT). You can do this with a meter, but I prefer to check it using the OBD connector, port 2, DTM 2 (see the FAQ) as it also checks connections and wiring, plus doesn't get my hands dirty.

Make sure the throat of the TB is clean of grunge around the throttle plate and the two little vacuum ports or there hoses aren't plugged. Also make sure the throttle plate isn't off centre and binding and the shaft isn't extended too far and binding inside the TPS. One crude way to test that would be with the idle stuck a little high, manually force the throttle fully closed to see if the idle drops like it would if you blipped the throttle. A better way is with the TB removed for cleaning and held to a light to check for an even band of light around the throttle plate when fully closed. Of course if the TPS was worn or set on the hairy edge, it would behave similarly, so do attend to that first.

You asked what happens if the TPS was disconnected. The ECU wouldn't know to control idle and the idle speed can float, anywhere from being unsteady to going high or going low and stumbling, even stalling. By tweaking the idle stop screw you could in theory get the idle close to being correct when the engine is warm, much like you've probably done.

The FAQ adjustment procedure for setting LH 2.4 base idle will work, but requires a decent tach and feeler gauges. Most of us can do a perfectly decent job without a tach just adjusting the idle stop screw using the 1/4 turn method and making sure the TPS click isn't set on the hairy edge.

For you interest, Volvo actually revised the LH 2.4 adjustment procedures to be more precise in a TSB using just feeler gauges that does not require a tach or pinching the IAC hose and risk splitting it. I recently posted the revised Volvo specifications here:
https://www.brickboard.com/RWD/volvo/1681369/220/240/260/280/throttle_body_adjustment.html
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now