Stevo86

I am not even going to attempt to argue with ya man but I have on more than one occasion fixed a vehicle's idle with the throttle stop screw not the IAC.

BTW OP I forgot to add that you may need to unplug it first but I'm going to hazard a guess that cliff's "how to test" is better than what I can give you.

cliffyk

I am certain you have, but most likely because it the screw tip had "eaten" into the tab on the bellcrank (common on higher mileage engines), someone had messed with it in the past or it was a new aftermarket TB. I have not found a single aftermarket TB that had the stop screw properly set.

The preset adjustment is not hypercritical, however it needs to be set so that at normal idle the duty cycle of PCM's signal to the IAC is somewhere between 30% and 45%. In fact the fastest/easiest way to set the stop screw is with an oscilloscope, just hook it up the the IAC wires and turn the stop screw until the duty cycle is in range. I have found 38% to 42% to work best...