Cody Garton

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.

Cody Garton

Also there are NOT any check engine codes.

Yosoyway

I have an 02 GT and the idle would go high and then drop to normal. I cleaned out the IAC but I didn't see much of a difference so I put in a new one and the problem went away. I also replaced the PCV valve at the same time. Good luck.

pure_kaos

Could be your TPS.

dawson1112

If the engine doesnt die when you unplug the IAC you most likely have a vacuum leak someplace. If you have a vacuum leak, it will also mess with the 02 sensors and cause the engine to surge like that. When you unplug the IAC it is closing so I know its working because you say the idle drops to normal. That means its getting the air it needs from somplace else. A stuck throttle plate, a broken vacuum hose, or the manifold is leaking from someplace.

tadpole1969

sounds like tps or like dawson said, i would
have it smoke tested

Cody Garton

Found the problem. It was the coolant temperature sender and switch. Replaced both and it runs like a champ. I appreciate all the help!