IAC questions: Test function, PIDs and IAC calibration

[turabo87]

I'm tuning a 4.8L swapped Fbody with a P01 PCM. It uses a drive by cable 90mm throttle body (Speedmaster 90 mm LS2 4 Bolt, PN: PCE138.1004) with IAC. I believe IAC is staying open and not adjusting for dynamic idle. How can I test IAC function and ensure it's actually moving position? What are some valuable PIDs to test IAC function, displacement and position? Also, anyone recommend a good stock compare file that uses a P01 PCM and drive by cable throttle with IAC from factory, that I can copy those parameters into it? Thanks.

PS: Using OEM TPS and OEM IAC for now. On a separate note, the OEM TPS has a HUGE deadspot between 0% to 40% with this TB, so I'm suspecting compatibility issues between the TPS half-moon female and throttle half-moon male. Wondering if anyone has dealt with this Speedmaster Throttle Body.

Thanks!

[Frost]

I've had this headache before... What I *CAN* tell you is that if you dig, you will see IAC commanded position and IAC actual position. What they call IAC actual position is absolutely NOT actual position, but rather a calculated value that is used internally by the ECM, likely to keep it from chasing tail over mechanical delay (PID "ringing"). Don't let that fool you.

Many aftermarket TB's have very poor IAC airflow in general. Once things are working and tuned warm, there may not be enough IAC left for cold start.

You can always start scanning while logging IAC commanded, use the scanner's bi-directional controls to force the IAC to some count that you want and see what it does to idle with the engine running. That will tell you if it's moving or not.