IAC check

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Just unplug it while the car is idling. If it struggles to maintain idle when unplugged, the IAC is probably fine. If no change when its unplugged, then IAC is likely the culprit.
If all mechanical settings and all sensors are as they should be
I would think your car should sputter around a bit or possibly die


You might have different results if the air bleed screw or throttle blade stop screw has been messed around with.

All three items above and believe it or not, even more, all work together to make up a system that attempts to give our Stangs a stable idle under all different kinds of conditions.

I have been struggling with a rough idle myself. However, I have an e303 cam. So that could be why. But I may try changing the IAC anyways.

I know it didn't die when I reset my idle and had it unplugged.
To tie 1TMF's and Grady's posts together (I know what you both are saying): Do the test on a colder motor (where the IAC is needed as part of the enrichment function). As Grady said, some folks (like me) have the hot, no-load idle set for zero DC from the IAC (so the test is invalid on a hot motor).

To test the IAC in its true dynamic, you can hook up an oscilliscope and check out the square wave - that's above what most of us want to do.

Good luck.