cbauer210

I have an 89 GT....fairly stock. The idle is always dead on if I am cruising around town. If I go on the highway for more than 15 min or so the idle gets very rough when I come to a stop. It surges and wont stop unless I drive around at back road speeds for a little while. What should I look at first to try and fix this?

Black89LX

-I would think to check the IAC... Clean it out real good.
-Otherwise go through the intake andignition system.
If you have been at near idle, RPM wise, while driving on the highway then maybe your plugs aregetting a bit fouled and you need to cleanthem out by driving on the backroads. (acceleration, and braking)

Does your temp come up a lot after driving on the highway?
What kind of speeds are you talking about when you say backroads? 45, 55
What do you mean by fairly stock? CAI, Exhaust, Heads, Cam...

Sorry for the questions, just trying to get an idea of what elsemight be causing this to happen.

cbauer210

By fairly stock I mean just a cat back 2.5" exhaust and a K&N Filter. The temp never comes up. It actualy stays very low at all times. When I'm driving around on back roads it is stop and go never realy going over 55mph at any given time. Usualy just driving aroung at 30-45mph between stop lights and stuff.

The problem has only started after getting off the highway for an extended period of time. I can drive on the highway for 15-20 minutes usualy with out a problem. It will surge maybe once or twice afterwards but then level out. If I drive for 30 or 40 min at around 65-70mph thats when it realy becomes an issue. Once I come to a stop it will surge prity hard and steady.

Twisted

You might be pulling oil through the pcv into the intake while driving on the highway (highmanifold vacuum). This might cause a rough idle until the oil is burned away. You could shine a flashlight through your throttle body to get some idea of what kind of oil is in there. Don't expect it to look pretty in there though...even if that's not the problem.