I see a set screw on the throttle body and the tps as well. How does one adjust with these two variables for proper idle?

Hi,
Sounds very random but I gather it’s almost dialed in to running nicely?

While you are under the hood be sure to check the accordion hose behind the AMM.
You might have a tiny pin hole in the volutes somewhere or the ends are not sealing under the clamps.

With the twisting of the engine a pin hole or slit can randomly open up and throw off the Idle mixtures.
Mostly this happens in town driving and the engines will get rough or out right die.
Use a cylindrical light up in the middle and flex it.

I haven’t followed but make sure the throttle plate and cables are not sticking and how old is the IAC valve.
Idle Air Control.

Check other vacuum hoses around the throttle body and down to where ever the evaporative canister is for emissions purposes.
It has a valve with two hoses and on 240s they rot.

Does your car have a Exhaust Recirculating valve?
If the EGR malfunctions it can cause idling issues but it will usually throw a code.
How is the CEL (check engine light,) anything?

Phil

Hi all , been a few months and I am pleased to say my 91 is running fine. Haven't done the muffler yet as the patches that I made are holding up. Regards
paul

Hi,

Glad to hear that kind of feedback on here. We need all the thread traffic we can get to help keep our entertainment levels up.
The cars may have diminished in numbers but I still catch glimpses of many going the opposite way on the roads.

It has taken a long time to killed off these repairable goodies.
You might consider getting stocking up on the exhaust system before those components slide in towards the antique category.
But then, ingenuity can prevail against many odds.

Phil

A quick 6 month update to say that the car is still running fine even though the muffler is a tad leaky , but something I can live with for now. The price for the muffler kit from cat back has gone up somewhat at FCPEuro. I just hope it will be available when I need it. The car has a tad under 230k miles now !

Basically, the computer automatically maintains the idle speed for you. All you can do is adjust the throttle plate closed stop screw (which I believe you are assuming is an idle adjust screw). Once that is set you can go ahead and adjust the throttle position switch to get the "clicking" to occur at the correct throttle plate position - don't try to adjust the switch before adjusting the throttle stop. Also, the manual suggests that you perform the resistance/continuity measurements at the computer but I could never understand why you can't take those measurements right at the switch connection unless you suspect bad wiring between the switch and the computer. Make sure you are working with a clean throttle body and clean idle control valve.

Quoting the Chilton Manual:

Base Throttle Plate Stop Adjustment

Loosen the throttle locknut.
2. Loosen the adjustment screw until the throttle is com-
pletely closed.
3. Tighten the adjustment screw until it just touches the
linkarm, then turn a half à turn further.
4. Tighten the locknut without changing the adjusting screw
position.
5. It may be necessary to loosen the throttle switch before
setting the throttle.



Throttle Position Switch Adjustment

1. Open the throttle slightly and listen to the switch. There
should be a click when the shutter opens, if the adjustment is
correct.
2. Loosen the throttle switch mounting bolts.
3. Turn the switch slightly clockwise.
4. Turn the switch counter-clockwise until the switch clicks,
then tighten the mounting bolts.
5. Recheck the setting.
TESTING
1. Tum ignition switch to the OFF position.
2. Check the throttle switch by connecting an ohmmeter
between ground and pin 2 on the control unit connector. ohm-
meter should read O ohms (switch closed).-
3. Depress gas pedal slightly, resistance should increase to
2000-3000 ohms (switch opens).
Connect an ohmmeter between ground and pin 3 connection on the
control unit connector. Ohmmeter should read
infinity. (full load switch open).
5. Depress gas pedal all the way. Ohmmeter should read O
ohms.
6. If specifications are not as specified, check at throttle
switch to see if fault is in throttle switch or its leads. Check
ground connections at intake manifold.

Thank you Chris. This is what I needed.

UPDATE: I removed the throttle body and adjusted the plate and TPS as outlined. After reassembly the start did not go too well. This is starting with a cool( South Florida) ambient temperature , about 70 degrees or so. The engine immediately died. I started the car again and gave it bit of gas and it held. I kept it like this till the engine warmed up . However once I let off it would grumble/vibrate, at a very low rpm 300-400 or so. While in this state I went to the engine and played with the electrical connection to the IAC valve and no change. Both hoses at the valve were secure and tight. I rotated the throttle pulley and noted that the humming sound associated with the IAC valve was fluctuating and this echoed with the engine vibrating in sync. I disconnected the hose from the air box and there was a strong pull when I put my thumb near the open end. I advanced the throttle pulley and noted a series of vibration associted with a suction , no suction at the hose end . It seemed to me that the IAC was opening and closing randomly at idle. I could actually hear suction come and go with a slight clicks from the IAC.
A while back I had purchased a used ECU to help solve an issue that I had. It turned out not to be the ECU then. If what I understand is correct the ECU provides power to the IAC once the TPS sensor is closed? The ECU then control idle rpms. So was it the IAC ( which I cleaned thoroughly) that was faulty or could it possibly be the ECU sending faulty signal to the IAC? I removed the the ECU and replaced it with the spare . The idle settled down steadily. Turning on the Air conditioning resulted with an uptick in rpms. No stutter or hesitation. There were no shaking after gassing up and backing off a bit. The transition was smooth. I drove around the block a few times , coming to stops , going in neutral and back in gear, turning off and on the air conditioning and everything seems normal at this time and I hope it stays that way. Now what I would like to know is if the IAC modulates during normal operation. The reason I ask is that even when the the revs are up there is still some suction at the input hose. I thought the IAC would close then but apparently it does not. Every IAC that I see for sale on ebay states for Regina systems which my 740 is. This leads me to think that it may not be an either open or close operation that perhaps some other system is. Thank you to all that provided information to help me solve this issue.

If the idle rpm drops down to 300-400, in fact anything below 700, then the ECU probably doesn't know the throttle is closed and it's supposed to be maintaining idle using the IAC valve (idle air). Best guess is the TPS (throttle position switch) is now incorrectly adjusted or the connector is not properly seated. With the engine not running, do you hear the TPS click just as you open the throttle spindle? It shouldn't be set on the hairy edge, but soon after the plate starts moving. Other issues with idle adjustment are the throttle linkage rod and throttle cable adjustment restricting the throttle plate from fully closing and clicking the switch. If needed there are OBD tests you can do at the diagnostic connector to check all of this out -see the FAQ here.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Hi Dave , thanks for your response. So , I had my son sit in the car while I watched and listened as he depressed the gas pedal and backed off.I do hear the TPS click in both directions. Upon closing , there is a tiny bit of movement after the click so it is not hairline by any means. Also the cable to the pulley is not tight and the throttle linkage is "loose" . I can easily rotate it with two fingers suggesting that there is not any tensions to not allow the throttle plate to properly close. After checking these conditions I started the car and it idled at about 7-8 hundred rpms. While at idle I turned on the ac and the idle increased a bit , as expected. Also going into drive / reverse there is an initial bog down ( just like when I turned on the ac ) of the engine with a subsequent rally of the rpms back to norm. I will be driving it a little more now and hopefully every will be fine.
Regard
paul

Just general info, Im not familiar with the regina system.

most cars have some means to measure airflow at idle, my 240 and 74 being bosche use an air mass meter, My Porsche 944 has an older "barn door" arrangement where a flap lifts and it has a rheostat which changes in continuity. Its a fairly similar Bosche system, but 85 , a bit earlier technology. Regina may be different again.

Point is the idle is usually very sensitive to any source of air which can enter the engine without being measured and accounted for, so any slight leaks may affect idle a lot. at wide open throttle the air fuel mix is probably less disturbed, proportionally.

I spoke ot a Porsche guy recently who described how he cobbled together his own smoke machine , he injected smoke, or something like it into the hoses , perhaps pressurizing them slightly while watching with a bright light for escaping smoke. he said he did find some leakage that he was not aware of , and fixed it which made his car idle a lot better.

maybe others have made up these "smoke injectors" or maybe they can be bought, not sure, others may know more on that. .

My Regina 740 has a MAF ( mass airlow sensor) in place of an AMM. Your suggestion of a smoke process has always intrigued me , moreso the fact that I maintain 5 vehicles in my household and probably would be a worthy thing to have. Today I made a few errands without any issues so lets hope it stays that way for now.

Well, the idle did not stay that way. I took the car on a 50 mile round trip and it ran well in cruise. However It shut off twice in slow moving traffic but started without issue. I got home , pulled into my driveway and then the idle started to stutter. I opened the hood and started to poke around at the various vacuum hoses and electrical wiring. When I touched wiring connection to the MAP sensor located near the drivers side strut ( a Regina thing!) I noticed changes in the idle. I will be loking into this shortly and report back. What a journey I am having !

I hope I am not being premature here but I think I have the idle issue solved? on my 91 Regina non turbo 740. Last year 2022 I put on a whopping 125 miles on this car only because it had issues with not starting like it use to. When it did start the runnning was terrible with backfiring stuttering and so forth. All this was solved by removing the accumulator/pulsation dampener...call it what you want...on the fuel sending unit. There is only one intank fuel pump on the Regina cars and this device was on the output side of the fuel pump. Since the fix I have been having idle issues. There were a few false positives but just recently I believe ( fingers crossed) I nailed the fix. After triple checking all vacuum and electrical connections the rpms would momentarily bog down every time I selected a gear or even turning on the ac, essentially when the engine was loaded. As a last resort , my patience running thin , I decided to swap out the ECU again along with a new MAP sensor. I was now reinstalling the original ECU , the one I deemed bad before , but before I did I cleaned the terminals with electrical contact cleaner. Now I am not sure if that did the trick or maybe readjusting the TPS and TB tolerances or just luck on my part. The engine since then is performing like a charm with no rough idle , no difficult starts.
At this juncture might I say thanks to all for your suggestions. Next task is to install a new mid muffler as the present one is ratlling like crazy and even has a few holes in its pipes.
Regards
Paul

the past couple of times I ordered th whole system from the cat back. I found then I could simply assemble the pipes and then lift it in and hang it , and then tighten all the U bolts..

every time I go thinking I can just change 1 pipe I find I'm in for a struggle

getting them apart can sometimes be a frustrating issue, because the muffler shops ( or maybe the factory) tend to squish the pipes up with the U bolts to a point where they become a difficult task to separate,, maybe need some cutting to free them up...

last time I tried to buy a piece of pipe on rock auto the price was ok but delivery for it was over $200 bucks so I got it local more cheaply. If you do an online order I'd check the shipping cost.

Always get the muffler shop to do the replacemnt,,Bolying it together always leaves a air inlet by the Ox sensor,,which wastes gas ^ screws your car way more down the line!!

Hi,

Glad to hear you made progress with the mysterious idle issue.

I was suspecting the throttle body area with air leakage.
The TPS and the throttle plate need to move repetitively to a minimal air allowance into the engines operation.
The IAC has limited capabilities or range of operation in idle mode per the TPS command.
The AMM mixture setting is “before the fact” of extra air slipping by. It does nothing correct mixtures in any mode. The ECU is looking at the exhaust sensor and RPMs and their alignment to the program lines.

So I’m saying, if you think you moved those other two settings then the computer is no longer out of sync to the throttle body.
Cleaning the ECU pins is not to be ruled out either.
So it’s good you are on top of that possibility and capable to have one on hand it seems.

Again, glad to hear it.
Thanks for the feedback

Phil

Hi Chuffsy,

Check the FAQ for "idle adjustment".

Happy holidays, Bill