I recently bought a 88 notchback automatic with 2.5l engine. Car had been sitting for about 15 yrs. The engine will only idle against the mechanical stop. No IAC control at all. Throttle body has been disassembled and cleaned. Tried a couple of used IAC, but no luck. SnapOn scanner says IAC is working, but it is not. Difficult to check with meter at the IAC. Any suggestions?
The IAC is a stepper motor. The ECM commands the IAC to move in steps. The ECM remembers how many steps it has commanded the IAC to move and remembers it's position. So when you scan it, the scanner will show where the ECM thinks the IAC is at, not where the IAC actually is at.
Take the IAC out. Hold it in your hand and make sure you hold it so the IAC can not over-extend the pintle and launch it across the garage.. Stick a finger in the IAC hole and feel for where the vacuum hole is. Then have someone start the engine and control the engine speed with your finger in the IAC hole. Close the hole more and the engine will run slower. Uncover it to make the engine run faster.
While doing this look at the IAC. Does the the pintle retract when the engine is running below idle speed? Does the pintle extend when the engine is running above idle speed. Both should happen.
Again block the IAC from overextending. It is fine to block the pintle's movement, that is how it is done in the throttle body.
Finally before turning the engine off, run the engine above idle speed so the pintle retracts. If you install the IAC with the pintle extended, the pintle will bottom out first and you will ruin your IAC by jamming the pintle into the IAC when you screw the IAC into the throttle body.