My car's been running fine for a while. I took it out the other day and it was idling fine when started. I took it on the highway and after I got off, it started idling high - around 1500-1800rpm. It's rumbling with small backfires (maybe just from idling high?). Anyhow, if I let the car sit after a run, the idle stays at 1500-1800. I started the car the next day and the idle seemed to be back to normal, except after a drive, it was back to the 1500-1800.

Before I go through an idle adjustment, what else could this be? I don't want to adjust the idle because clearly the car knows where "it should" be idling when it's started from cold.

What things should I check?

Thanks a lot for any ideas! -Joe

 

Mod list? Any emissions removal/vacuum reduction?

 

Hi Arghx,
Thanks for the reply!

Haven't moded anything recently but I have dp, catback exhaust and intake (stock ecu). Vacuum lines are all intact - still pulling strong at idle and all of the gears. Emissions equipment is all still on it (other than pre-cat).
-Joe

 

Sounds like something is changing after heat soak. You may want to check your ground wire contacts. They are sensitive to heat changes in the engine bay. May be triggering a false sensor reading that is upping the RPMs.

 

Wild guess: stud expansion as engine heats up is loosening grip on some intake manifold gasket, which then leaks air in, raising idle speed.

 

Thanks for the replies! I'm heading off for the weekend but as soon as I get back I'll check a bunch of things. I'll check intake and any sensors I can think of. I'll also check electrical as I know my negative connection needs replacing. I'll update after the weekend but if anyone has any other ideas, let me know before I get to work! :P

Thanks, -Joe