The car will start fine and come off fast idle perfectly and idle around 900 as it should, but as soon as you start driving the idle will vary between 1500-2000 rpm. The car is a '88 GTA with 305 TPI and a 5-spd tranny. I cleaned and reset the IAC. The SES lite is not coming on. What else should I try? It has a hypertech chip and 160 degree thermostat, would that effect the idle?

thanks!

 

Possibilities: Sticking throttle body, IAC faulty when warm, faulty computers/sensors (computer thinks the idle is lower than it is) when warm, or something is leaking vacuum only when warm. While expensive, your best bet is to get a scanner of some kind so that you can tell what the computer is thinking when the RPMs start to rev up. The answer will be obvious. Otherwise it will be a trial and error kind of thing.

 

Sounds like something is sticking or binding. Get a can of carb cleaner and spray down the moving parts of the carb (throttle shafts, etc) and see if that helps.

 

thanks for the help, I had the thottle body off twice. Everthing is clean. The IAC housing is the only thing I couldn't take apart, three bolts were already broke off, and I broke one more trying, so I left it alone. I just sprayed it good. A vacuum leak is a good possibility.

 

Might be wise to check that the CTS (coolant temp sensor - the one in the thermostat housing) is still good as well. IIRC, when that sensor reads a certain temperature, it allows the engine to go into closed loop and run off of what the engine sensors read, as its supposed to once its warmed up. If that sensor doesn't read properly (ie. too cold) it will never come out of open loop.