I have an issue that maybe some of you can help with.

1987 244 with unknown mileage. When I crack the throttle quickly (car parked), the engine bogs down real bad and will backfire through the intake. Slowly raise the throttle and the engine runs nicely.

Idles great and runs clean. Car starts easily, even at 20 degrees. Compression is 175 across the board. Fuel pressure tests are all good (Pumps replaced about two years ago). New fuel filter, amm, plugs, wires, cap, rotor.

Verified that there are no vacuum leaks anywhere, including the brake booster.

Fuel pressure regulator is functioning properly (bumps from 30-40 psi when disconnected).

I have monitored the O2 sensor signal and injector duty cycle and everything looks great. When I crack the throttle, the duty cycle will increase sharply and the O2 sensor responds throughout the entire speed range. TPS, coolant temp sensor, etc. all check good. Timing belt is recent and cam timing is dead on (I checked again last night. Alternator voltage is good. Overall, I cannot find anything wrong with the injection system. I even tried a couple different ECUs with no change.

There are two possibilities, but I need more opinions before I spend more money.

Injectors could be marginal. Perhaps they are not flowing enough for max load. If they were partially clogged, I would expect a greater duration at idle to compensate. I am getting about 5% duty cycle at idle when hot.

I get no change in advance when I remove/apply vacuum at the diaphragm on the ignition ECU. I know that is not normal, but I have run 240s with that port blocked before and never noticed a problem.

Is the ignition supposed to advance, or retard, on acceleration. The timing does advance with engine speed, so that part works. If it is supposed to retard, how would that make it any better (other than for emissions purposes)?

What do you think?

Anthony

Well, it was the injectors. I put on a spare set that was given to me and the stumble and backfire is gone. It is obvious that they were not supplying enough fuel for full load conditions. The duty cycle at idle went down even more from the originals, so there was a definite improvement.

I can crack the throttle to full open and I cannot get it to backfire. Also, the car will pull pretty well from 25 mph in 4th gear (although not very rapidly). So it runs better than it did before. I would say that probably has to do with the failing amm that was in there before.

I do not know about the lack of vacuum advance/retard, but I would not feel bad about buying an ignition ecu if necessary. I know that applying a vacuum to the diaphragm does nothing. At this point I do not feel that it is necessary.

The only question left is: Does the ignition advance or retard on acceleration?
I am referring to the vacuum portion, not "centrifugal".

One other interesting point. I disassembled a bad vacuum advance diaphragm from an old v/c ignition box. It is actually a variable inductance device. Attached is a either a ferrite or powdered iron slug that moves within a core.

I thought that was interesting. A 1000 times more reliable that a variable resistance.

Thanks again for all the input.

Tell you what, I'm still a bit skeptical. Four injectors with impaired flow? Never seen nor heard of it. Of course there could have been some attempt to clean the fuel system with Karo syrup, perhaps, as one can imagine many technician induced reasons for this. Or the wrong injectors...

Can't argue with a working car though. One suggestion. Sign up. Post your duty cycle readings for reference. Tell us how you measured them (meter model, etc.).


--
Art Benstein near Baltimore

Dr. Epstein was a renowned physician who earned his undergraduate, graduate,
and medical degrees in his home town and then left for Manhattan where he
quickly rose to the top of his field.

Soon he was invited to deliver a significant paper at a conference,
coincidentally held in his home town. He walked on stage and placed his
papers on the lectern, but they slid off onto the floor. As he bent over to
retrieve them, at precisely the wrong instant, he inadvertently farted. The
microphone amplified his mistake resoundingly through the room and
reverberated it down the hall! He was quite embarrassed but somehow regained
his composure just enough to deliver his paper. He ignored the resounding
applause and raced out the stage door, never to be seen in his home town
again.

Decades later, when his elderly mother was ill, he returned to visit her. He
reserved a hotel room under the name of Clark and arrived under cover of
darkness.

The desk clerk asked him, "Is this your first visit to our city, Mr. Clark?"

Dr. Epstein replied, "Well, young man, no, it isn't. I grew up here and
received my education here, but then I moved away."

Why haven't you visited?" asked the desk clerk.

Actually, I did visit once, many years ago, but an embarrassing thing
happened and since then I've been too ashamed to return."

The clerk consoled him. "Sir, while I don't have your life experience, one
thing I have learned is that often what seems embarrassing to me isn't even
remembered by others. I bet that's true of your incident too."

Dr. Epstein replied, "Son, I doubt that's the case with my incident."

"Was it a long time ago?"

"Yes, many years."

The clerk asked, "Was it before or after the Epstein Fart?"

Were all four injectors impaired? I do not know. I did put in a set of good used ones. They came from a 1990 that I sold to a friend a long time ago. I cannot remember why he changed them, but I do know they did not solve his problem. Either way, I knew they were good.

I have no doubt that the injectors were at fault. I did not touch the intake gasket yet. I did not disturb anything else, so it is not likely that anything was accidentally "fixed".

I anticipate being asked about the two ground wires that are secured with the injector rail. Yes, they were checked, cleaned, tightened before and were not the cause of any problems. I did not swap the rail, or the fuel pressure regulator with it. etc....

Duty cycle before change was about 5.5% at idle. After the swap it runs about 3.8-4.0%. It is a Sunpro digital multimeter that I bought at least 5 years ago for auto use. I do not remember the model# off the top of my head.

Thanks for reporting the numbers.

I've had this scope handy in the garage, and for years grabbed it to better understand what engine management was doing. Most of that time, I also had this relatively inexpensive multimeter, completely oblivious to its "duty cycle" setting. Once I did discover it, I got a chance to verify its accuracy, and be impressed by what a $30 DMM can do these days.

Having some numbers for a performing car could help many others who don't have old tach-dwell meters kicking around but do have access to one of these inexpensive multi-function digital meters.

As you can imagine, a number like 3.8 - 4.0% is much easier to compare and communicate with other DIYers than poorly documented scope traces.


--
Art Benstein near Baltimore

If speed counted, rabbits would rule the world.

I actually have scope in the garage too. I am an electronics technician by trade so I do understand it. However, the meter is much more convenient. My meter was not that cheap though. I think it cost me about $90-$100.

I like the scope for looking at the ignition. It can catch things that nothing else will.

Hi ART,

I am with you on having four injectors at fault. I can go with one questionable injector or o-ring seal, fixed by four “fresh” ones and fitting correctly!

His other question about timing advance under a hard acceleration is something I have not thought about in a long time.

I think it has to advance more degrees BTC in order to match engine piston speeds and to give more time for the added fuel/air mixture to ignite.

The ECU map and the knock sensor play a part in that.

Probably the reason for the diaphragm to disappear in later years as it did not work as well as the better fuel management systems. They would probably like to get rid of the FPR but there are too many variables for a proper volume of fuel delivery.

Of course, like the o-rings I do not know how “fresh” my memory is on this either.
Phil

Hi Phil,

Yes, I forgot to comment on the ignition. I believe 85-88 cars were the beginnings of NOx-reduced spark control. This list has known for 10 years or so to plug the vacuum line on these V/C or "computerized" ignition controllers to help pass a flunked NOx test.

OP brought up a very good point about the reliability of the load sensor. It does indeed use a diaphragm, however the diaphragm's load is negligible, unlike that of a vacuum advance on a distributor. All the V/C's diaphragm needs to do is withdraw a ferrite core from an inductor, just like your old automobile AM radio was tuned. No mechanical load to speak of, and the inductor provides a smooth translation of motion to something the microprocessor understands.

Regarding the injectors, I've heard folks claim great and wonderful solutions merely by changing the o-rings on LH. I've never seen that for myself, and as with the injector being at fault, remained skeptical despite the reports. I must admit, where I live, rubber lasts a bit longer than it does in Sunny SoCal, so I'm willing to buy dried-out injector o-rings could leak enough air to backfire.

Here's an advance curve limit spec for a V/C ICU used with B230F engines. The left graph uses RPM on the x axis, and note the y axis indicates variation, not absolute BTDC advance. The right graph's abscissa is read in vacuum, and you might have to translate the metric in your head to see where the expected 19-20 in. in your manifold hits the wild part of that curve. Making that non-linear response (for emissions) and setting an overall limit is a lot easier in the computer, than it was in past distributor-mounted arrangements of two diaphragms for advance and retard.

Like you, I was hoping to find the fuel pressure regulator got modernized at some point allowing computer control -- because that would mean cheap and abundant fuel-compatible electronic pressure transducers could be found in my pick-n-pull. As far as I know, they are still of the same design yours and mine are.




--
Art Benstein near Baltimore

REAL men don't need voltmeters.

"New fuel filter, amm, plugs, wires, cap, rotor."

All of these items replaced in pursuit of this "new" symptom? Especially curious about the "new" amm. Care to say a few words about the onset of the symptoms; history of the car?

"I have monitored the O2 sensor signal and injector duty cycle and everything looks great."

This implies you know what you are doing, compared with the average DIY mechanic. You know the duty cycle will remain constant without the AMM output to add load into the fuel map. But I take it you aren't entirely confident watching a multimeter's indication of duty cycle (my assumption) confirms the AMM's response time or dynamic match to the airflow. The O2 sensor will not enter into this in a throttle blip or (while driving) the need to merge.

There are two possibilities, but I need more opinions before I spend more money.

I would discount injectors as the problem. Also, the load portion of the V/C ignition timing -- I agree from my experience (with V/C ignition) RPM advance alone still leaves you with a pretty zippy car.

My thoughts are focused on unmetered air if the AMM can be trusted.


--
Art Benstein near Baltimore

To write with a broken pencil is pointless.

I should describe the history a bit better.

Car belongs to a friend, but has been in my care for a while. Yes, it has sat for several months. Over a short period of time, it became very hard to start. Actually, it would start easily but immediately die. That was easy, disconnected amm and would start and idle. Ecu would default to about 10% duty cycle at the injectors with no amm. Once warm, it would idle on that amm, but O2 sensor voltage would go right to nothing as soon as the throttle was moved off idle. Base mixture control had to be almost full rich before I had any indication of mixture control (the usual .5v crossings of O2 sensor voltage)at idle. Second amm (used): No start, would not function, even on warm engine. Third amm (also used):overly rich, no matter what.

Bought new amm from fcp. Starts easily, good mixture control, maintains good mixture control from idle to over 3000, assuming I do not crack the throttle too quickly.

I feel confident that the new amm is working properly.

The duct from amm to throttle body is perfect. Not a hole, crack, abrasion in sight. I could try the intake gasket on the chance that the vacuum leak is not large enough to find at idle. I am using the carb cleaner method to try and find leakage. It has always worked before, but then again...

I appreciate all of the input.

Sounds like you have my questions covered. I would add the fact I have not yet read any well-documented success using any carb-cleaner, propane, or water spritz method of locating vacuum leaks on a computerized, idle-controlled motor. However, I've not tried it that way since the days of simple (non feedback) carbs, so I cannot knock it as a method, having found any leak such as that below using a stethoscope.

But the leak you see below was partially indicated by sooty 2,3, and 4 plugs, as well as a fluttery reading on a vacuum gauge.


--
Art Benstein near Baltimore

"You can observe a lot just by watchin'."

I cannot remember if I disabled the idle control when I tested. I will have to try that again and see. If I did not, that would explain some things. I am going to change the intake gasket anyway, I think.

"I am going to change the intake gasket anyway, I think. "

Whenever I have done this, I've felt it was a move postponed too long. There are few parts I will change merely on mileage or age, but my brief experience shows these things are brittle after 20 years.

A smoke test would be best, but that's a diagnostic I haven't yet built.

Another whack at trying to catch a whiff of propane (or carb cleaner) might be to leave your DMM set to duty cycle and connected across half of the idle valve, but your LH2.2 base idle test point sets a fixed duty cycle, so that might work, and you could simulate a leak to test that theory, or explore the bounds of the idle valve's control range.
--
Art Benstein near Baltimore

The math professor went crazy with the blackboard; he did a number on it.

Have you cleaned the throttle body and idle air control valve? Make sure all vacuum lines and fittings allow air flow.
Dan

Yes, they are clean enough to eat off of.

Are you sure the AMM is doing what it is supposed to do, the symptoms sound like the limp home mode of a failed or failing AMM?

What is the condition of the engine harness?
Dan

Hi,


You have an intake leak. Your engine is running lean. I deal with lean running every cold day, because I burn E85. E85 does not vaporize well until the engine warms up, hence lean burning.

Have you ever replaced the intake manifold gasket?


Goatman

You might check these things.

Have the throttle body clean and the plate adjusted as tightly closed as possible.

Check the operation on the throttle switch adjustment as it has to tell the ECU that you want to come off idle and pull away. Look at the general condition and for corrosion in the connector to the switch.

The IAC has to close while setting BASIC idle in order for it to operate in the middle range of air control. This applies if you have a black knob on your throttle body for setting bypass air. The IAC shuts on these 86-year cars by grounding a white wire with the blue stripe located in an open-ended connector above the AMM. Leave the other wire in it alone.

Phil

The TPS is adjusted properly and presenting the correct signals to the ECU. The idle is adjusted to spec by grounding the test wire. No problems there that I can detect.

I have checked as thoroughly as I could for vacuum leakage. I have sprayed carb cleaner in every spot imaginable with no indication of leakage. I even blocked the ports for the brake booster, charcoal canister, cruise control, and climate control with no improvement. I have already replaced the hoses and rubber fittings for the crankcase ventilation and IAC valve.

I have NOT changed the intake manifold gasket, although I do have one to put on. Is it possible that the leakage is not enough to show at idle, but be a problem on acceleration? I guess I can change the gasket and see what happens.

n3K,


You’ve already eliminated a large number of factors. Do be aware though that unless you used OEM parts for the crankcase hoses and idle valve, you may find these hoses cracked in short order. Mine cracked wide open in less then 6 months. I believe the brand was Üro parts.

The OEM hoses are expensive, but worth it. And, if the manifold gasket is original, then it wouldn‘t be a bad idea to change that also. Injector seals are less likely to leak, but if they have been disturbed, I would not count on them retaining their elasticity.


Goatman

I actually replaced all of those hoses last week. I will probably go ahead and change the intake gasket.

you didn't mention the large Bellows from the AMM.. Any signs of Cracks in that 'hose'?

A crack in that would usually cause an Idle issue. I toss that out there since you've seem to really gone through everything.

--
'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.lyse.net/brox/TonyPage4.html http://cleanflametrap.com/tony/

Sticky intake valve; or poor adjustment.

Cat conv burnt out - too much exhaust flow.

Bad coil. failed plug wire - I know you said they were new. Failed spark plug.
--
240s: 2 drivers & parts cars

Sounds like a blocked catalytic converter. Have someone rev the engine while you hold a folded rag over the tailpipe. You should feel a very strong exhaust stream with rpm. If not, the converter is plugged.
It isn't injectors.
It isn't ignition advance.

I did check and it seems fine to me. The engine will rev strongly once I cant past the point that it stumbles. It does not seem like the exhaust is restricted. Maybe partially?

Thanks

On marine Volvo engines the timing belts will slip a cog or two. It might be worth confirming that the cam timing is correct.

Yes, I did check and the camshaft timing is right on the money. The belt has less than a few thousand miles on it, so I would not think it is stretched. The tensioner was also replaced at the time.

Describe this backfire through the intake more if possible. Usually that indicates lean or ignition cross-firing ( or blocked exhaust...)