So I've been having a weird idle issue as of late. The car will only idle when I slowly let off of the accelerator to bring it gently to 840rpm-ish. If I'm driving the car and I press the clutch in without babying the idle down to the correct rpm the car will die. Sometimes the car will try to keep itself alive when I push the clutch in/let go of the accelerator without babying it; The car's rpm will drop to ~300rpm, skyrocket back to ~1400rpm, and either die or settle itself back down to 840rpm-ish. I've replaced everything that I can think of that would cause this, which I will list below.

- New and working ICV
- Tested and working TPS
- DME is working (swapped with another 944 s to test)
- New coolant temp sensor (to dme)
- AFM was replaced not long ago, but could have gone bad? I haven't tested it yet, but I wasn't sure if a bad AFM would cause such an issue. (car idles perfectly with AFM unplugged, but won't drive obviously). I also noticed that the AFM door seems loose, for example, when I push the door in and let it slam shut, it bounces against the stopper twice.
- New FPR, Injectors, Fuel filter, Fuel pump, Fuel Dampener
- Vacuum tested the car and found no leaks besides dipstick hole. (plugged the hole to test if it was the issue and nothing changed.)
- New 02 sensor, but could be an issue? With the 02 sensor unplugged the car will not die, instead the idle will jump up and down until it settles at correct idle speed, not sure what this means.
- Speed and Hall Sender good (tach bounce while cranking, verifying that speed sensor is good)

I'm at my wits end with this car. If anyone has any clue what the issue could be please let me know, thanks!

 

Did you re-track the arms on the AFM? Did you test the AFM for dead spots? The most common symptom is hesitation//stumbling while driving, and likely not related to idle issues, but still a good thing to check.
What is your fuel pressure?
TPS "clicks" as soon as throttle is actuated? The TPS sensors on these (non-turbo) cars are binary (idle, and then it clicks as soon as throttle opens to indicate non-idle condition)
Did you test/replace the DME temp sensor (blue top sensor) or the gauge sensor? There are two separate temp sensors.
110% sure you have no vacuum leaks?
O2 sensor possibly related if bad. Unplugging causes DME to run in open loop mode (does not adjust fueling dynamically and only uses preset fuel maps)

"Surging"/fluctuating idle usually indicates a vacuum leak.

I would check your TPS - if its not clicking as soon as the throttle plate opens the DME may not know the car is in idle mode when taking your foot off the gas pedal. For instance, when you put in the clutch and take your foot off the gas, it should trigger the ICV to start modulating idle speed. If TPS isnt working/telling the DME that the throttle plate is closed, it may not be activating ICV and I believe the S models (and S2) have their idle speed fully controlled by the ICV (no throttle adjustment screw).

 

MY 88 944s had similar issues. It ended up being TPS. Verify with ohm meter, reference Clark's garage for test method.

 

AFM is new.

Not sure what my fuel pressure is, going to test it this weekend. I don't think its off though, as the car runs good in all other rpms.

Yes the TPS is properly adjusted to click into idle. I tried unplugging the TPS and seeing if anything changes and the car just constantly surges, it never rests back down at 900~

Yes the DME temp sensor is replaced and working.

I smoked tested the car for vacuum leaks and saw a faint faint FAINT haze of smoke, but no identifiable location after checking all of the vacuum hoses (which have all been replaced too).

When the 02 sensor is unplugged the car runs rich, when plugged in it seems to be running at a proper AFR. I don't have a wideband so I can't know for sure though.

I know the ICV is working as it is new, it "buzzes" when the key is turned to ignition, and the car will not idle with it unplugged.

 

Just want to verify a few things here, so forgive me for asking again

-AFM is new? As in a brand new one or a "working" used one you purchased? If its used, test it (procedure on clarks). Bouncing against the stopper is normal - I think mine did the same.

-Are you sure you changed the DME temp sensor and not the gauge sensor? DME temp sensor is the easier to reach blue top sensor near the front of the head (I assume you know this, but again just checking). Gauge sensor is under the intake.

-You said the ICV is new, that means brand new part correct? Note: The 16V cars idle is controlled by the ICV - which opens and closes to adjust the amount of air bypassing the throttle plate to keep the idle at the target RPM. The ICV on these cars is different than the 8V cars (binary and not dynamic) so just wanted to double check you have the correct *new* ICV for a 16V car - if used I would be sure to give it a thorough cleaning and test with 9V or 12V source. Also verify the vacuum lines running to/from the ICV are intact and sealed as well as at the throttle body and intake connections.

-Idle surging (albeit you said only when the TPS is unplugged) does point to a vacuum leak typically caused when there is unmetered air entering the system outside of the ICV which then tries to adjust resulting in a up/down type of hunting. You mentioned you saw some smoke when doing the last test, I would try to investigate that further. There is a vacuum line that connects to underneath the intake that is easy to miss and can create a large vacuum leak. Of course this could also just be a by product of disconnecting the TPS.

-Definitely curious about your fuel pressure. People always make assumptions on this and think because they have fuel pressure that all is well, but a bad FPR can cause all sorts of issues (as well as leak fuel into your vacuum system) and its impossible to verify without a fuel pressure reading. You can also remove the vacuum line running to the FPR and fuel damper and see if there is fuel in the vacuum line (if so the diaphram has torn). Low pressure could be an issue with the pump or an obstruction (filter or fuel tank strainer). A high pressure reading typically points to an issue with the FPR.

-And just to double check, you tested the TPS with a voltage source correct? Not just listed for the click? I might do that if not done already to be sure the TPS is good. Clarks might have some info.

The main things that I can think of that would affect idle are vacuum leak, TPS, DME temp sensor, FPR (fuel pressure test), possibly AFM (less likely imo), possibly O2 sensor and lastly, bad DME computer (last resort sort of thing though because its hard to test). FWIW I do have a 944 S2 DME computer I would be willing to lend out for a deposit, but unsure if its compatible with an S.

 

AFM is new, the symptoms of the issue seem to point to something down the line, not the AFM.

Yes, the DME temp sensor is replaced. I used to have a very rough cold start issue that was fixed by replacing the sensor.

ICV is new, working, and correct Bosch part number.

Today I drove the car to a shop my buddy works at and smoke tested it again, found a massive leak that didn't come up last time for some reason. It looks like its coming from either the air/oil separator or a vacuum line behind it. I tested my TPS and idle is clicking into place correctly, but the WOT isn't registering. I have it correctly adjusted, so I'm just going to replace it while I have the intake off to fix the vacuum leak.

I'll keep you all updated on if the vacuum leak and TPS end up being the issue. Thank you for taking the time out of your day to help me locate the issue, its much appreciated.

 

My money is on the vacuum leak being your issue

The TPS is a very simple device - if you carefully crack it open you will see how it works. There is a curved metal spring in there that gets bent/worn over time and can usually be fixed just by slightly bending it back to original position.
https://www.clarks-garage.com/shop-manual/fuel-06.htm

 

Update:

After smoke testing the vacuum system I located a fairly big leak coming from what looked like the aos or the vacuum line behind it. I took the intake off and checked the vacuum line and it was good, but the AOS was leaking from the seal that keeps the two pieces of the AOS sealed. Replaced the AOS with a non leaking one, replaced the O rings, and put it all back together.

After starting the car it still has the exact same issues as before with no change. What do I do and where do I go from here? I'm so frustrated and fed up with this car and I have no clue what to do next.

 

And did you test and/or inspect/open the TPS?

 

I tested the TPS from the DME Connector under the passenger side carpet. I followed the Clarks Garage instructions and everything checked out besides the WOT test. With the throttle in the closed position the multimeter read infinite Ohms, however at 2/3 throttle there was no reading at all. Only when full throttle was applied was there a reading, and it was 0 Ohms.

Previously, more than a year ago, I replaced the throttle body seals and when I took the TPS off I noticed quite a bit of oil in the TPS. I let it all drain out and reinstalled it. Would this have fried my TPS?

Planning to take the TPS off today and test it and look for oil.

 

When I tested my TPS, it was also filled with oil. I replaced it with a used but bench tested good unit. I bench tested the old unit, it would not register on the ohm meter. I cleaned it with electric cleaner, many times and it now does register on the ohm meter for the idle position...it's now a spare.

One thing to notice is that the throttle response back , from high rpms to low is much faster now.

The oil issue happens over time based on the oil breather / vent.

 

Its important that the TPS is working properly so the DME knows you are at idle and it will use the idle fuel and timing tables, and when at WOT to switch to WOT timing and fuel tables. In between those two areas it uses closed loop part throttle settings. The TPS should have two different contact points - one for idle and one for WOT, and in between those two DME knows its for part throttle.

You also must be sure the TPS is adjusted/clocked correctly, so that it clicks (changes from idle to part throttle mode) immediately when the throttle plate opens. Ensure that this is the case.

 

Update:

Removed the TPS and tested it and noticed that there was no reading on the ohm meter for the idle click. So I ordered a new TPS.

Installed the new TPS today, and it did fix a small issue, but the car will still not idle. With the new tps, my throttle response is a little bit better, and no more stuttering when accelerating. However, the car still has the same issue as before, no idle. Whenever I take my foot off of the accelerator the car dies. Before anyone asks, yes the TPS is properly adjusted, there is an audible click as soon as the throttle moves from the TPS.

What should I do next? Last time I smoked tested the vacuum system I only found one leak, which I fixed (AOS seals). Should I smoke test the car again for vacuum leaks? Could it be symptoms of a bad AFM?

 

Verify the ohm measurements of the new TPS. Also verify it at the dme. Does the isv open and close quickly with voltage applied?

Lastly do another smoke test to ensure you are not fighting a vacuum issue

 

I think a bad AFM usually exhibits symptoms of stuttering throughout the RPM range and not poor idle.

And, if I recall, the AFM isnt actually used at idle, since the car uses open loop for idle/WOT (runs off of preset timing/fuel maps and doesnt use inputs from the O2 sensor or AFM for fuel adjustments).

It still seems like a vacuum leak, DME temp sensor or ICV related issue.