IAC valve issues
 

I recently purchased an 1996 GT/ 5 speed with 54,000 miles on it. Drove it for about a week with 0 issues. Now all of the sudden the car idles high (around 1800-2000 RPM). Car will crank, but has issues starting. When it does start and I do get going sometimes the car will violently jump in 1st gear around 2500-3000 rpm. I've cleaned the throttle body, did nothing. I checked for vacuum leaks, and didn't find anything obvious. ( first mustang so I could be missing something.) I read a few different things about the IAC valve. I think that could be my issue with the high idle, but it doesn't explain my car jumping. I have the IAC with the black vent on top, and when the car is idling and, I unplug it, the idle drops to a normal level. I need some help from the Mustang gurus of the forum.

I'm kind of thinking maybe the throttle position sensor. Only because I had a Lincoln Mark VII once, with EEC-IV mind you, that would all of a sudden while cruising would speed up and even backing off throttle the speed would remain. The TPS was defective. Also have you tried cleaning the MAF sensor as well?

Cleaning the MAF is my next step. I didn't have time to pick up MAF cleaner today. Hopefully that helps .

Found the problem. It was the coolant temperature sensor and switch. Replaced both and it runs like a champ.