Introduction and Idle Problem (Long Post)

Hi everyone! I am finally back to working on the bird, and it is still misbehaving.

I have replaced the upper intake gasket and the gasket between the throttle body and intake, and the ECT (I changed all the hoses and flushed the system while I was at it and checked that the intake cooling passages were clear). This still did not clear up the problem.

I can now replicate the idle issue at will, whether the car is cold or at operating temperature. As long as the RPMs do not drop below about 1000 the car will idle correctly all day long. If it is at idle and you slowly let the clutch out (without touching the gas pedal) and drop the RPMs to less than approximately 900 the IAC seems to overcompensate and bring the idle up to about 1200 RPM and hangs. If you do it again it will recover up to around 1500 RPM, where it will stay. When this happens I will sometimes get a CEL with the code 42. A rev up to about 2500 RPM will clear up the CEL. If you unplug the IAC idle drops to around 750-800 where it is set. If you plug the IAC back in it will idle to 2000 and stay there until you turn it off and back on again.

I also put a known good computer and VAM in (again) with no change. I can unplug the VAM and still cause the same high idle. I disconnected the alternator completely to see if it was causing it and it did not help. I even checked the diode near the IAC and it is good.

I think my car hates me :-(

I have the Breakout Box connected and even isolated the computer from changing the timing and it still did it. Is there anything else I can check?

The wanting to stall when it is cold and you tap the gas and rev to about 2000 RPM is also still there. Maybe another piece of the puzzle? Also when you rev it up to 2000-3000 RPM the idle will drop quickly to about 1500 and then slowly go down to 1000. None of my other Turbo Coupes ever did that. They would quickly go to the idle of 1000. This car does not hunt for an idle, it seems to pick one and does a remarkably good job at keeping it there.

The orange O2 ground is on the turbo. I have not tried replacing the FPR.

Any ideas (besides burning the car to the ground in the back yard!)? I am going to try to borrow a scope from a friend to check the waveform at the IAC to see if that tells me anything.

BTT Any ideas?

Thanks!

Have you checked the check engine light codes? There may be more in there.

The only code that shows up is the 42. That code has been popping up for almost 100K miles. Two VAM's, 3 ECMs, new IAC and 3 O2 sensors later it is still there. It will go weeks before that code shows up and it is transient. If you clear it after the CEL comes on (always only for a few seconds)it will not be back for weeks, sometimes months. That is what has me stumped. The car will act up idling and the ECM thinks everything is OK. Aside from the idling problem the car runs great.