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Re: IAC doesn't do its job
I suspect you have a vacuum leak somewhere.
It could be thru the PCV (a controlled leak around the throttle blade) or other where you have unmetered air entering the intake.
Anyway...the IAC is attempting to respond to the growing distance between the falling rpms and the "target" ERPM value....and so it essentially opens the throttle to increase speed, which brings enough overall air flow so that the leak is masked/less significant. And the actual ERPMs are over the target value so its time to close the "throttle". And so this progresses thru time without any "damping" to smooth it out leading to your rolling idle.
If you can download your fuel-trim values (I forgot the official acronym for now) which is an integration (average) over a long period of time this may confirm or dispel the above conclusion.
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1988 Chevy R-30 1 ton DRW pickup (217k)
1991 Chevy S-10 4WD pickup (192k)
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2002 GMC Yukon (185k)
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