6xStang

In warmer weather, my '89 5 liter motor will suddenly jump up from a 700 rpm idle to about a 2500 rpm idle, and stay there. If I shut it and restart it it is fine for a minute, then it hops back up without warning. if I start to engage the clutch with no throttle, it will fall back to the 700 rpm idle. Car runs fine in all other aspects, and has passed emissions inspection.

It doesn't do it so much in colder weather. I've cleaned and then replaced the idle bypass valve; no difference. I've disconnected the battery for quite some time, then reconnected. No difference.

OK gearheads, how do I troubleshoot and fix this puppy? Due to the temperature dependency, I am thinking MAF sensor? How to test? Someone else had suggested TPS. How to test?

I have no fancy diagnostic machines and hate replacing parts that don't need replacing.....but I might actually have to go to a shop if I can't get some good thoughts from this august crew. Or, I might just have to get a 2006 GT.....

RumbleBee89gt

hey, im havin around the same problem, let me know when u find what was wrong!its anoying huh!?late

ekthrilla

check and make sure your IAC valve is ok. if its bad it will do the same thing your talking about.

6xStang

I've cleaned and replaced the IAC (Idle Air Control=Idle Bypass=Idle Air Bypass). Though I replaced it with a used one, the problem was completely unchanged from one to the other). It does really seem like it should be an IAC problem. Maybe I should just bite it and try a new one before racking my brain further.

Both valves had been thoroughly cleaned with carb cleaner......