Diamond 944

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!

walfreyydo

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).

GG944s

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

Diamond 944

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.