Intermittent high idle

[baldjohnny]

'91 YJ 4.0 5 speed- When coming to a stop and depressing the clutch, the engine will immediately idle at anywhere from 1200-1500 rpm. It'll hold that for a few seconds and drop down to normal idle. Sometimes it drops down sooner, sometimes a little later, but it always reverts to normal rpm.

Replaced (plug n' play type of guy):TPS and IAC, pulled/ cleaned the throttle body IAC slots, and lubed the throttle cable. I know a vacuum leak could be a cause but if there's one there I can't find it.

Could a faulty injector hanging up cause this? I'm a little stumped as to the cause, and wondered if anyone had any suggestions?

 

[Sine Deviance]

I just swapped a 4.0L TB onto my 2.5L and mine is doing the same exact thing. It'll idle around 1400-1500 when I come to a stop and it'll gradually idle down to 900-1000 (which is still a little high, it should idle around 800.)

To me, it's like the ECU thinks the engine is cold and is bumping up the idle to warm it up.

 

[baldjohnny]

I figured changing a couple of things out (TPS/IAC) wasn't going to hurt anything; if they hadn't been changed before it had 137,000 on it and would need it sooner or later. It idles perfect once it drops down, no bucking or jumping. Think I'll put a new throttle cable on it just to say I did!

It's a little irritating, but that's okay. By the time this gets worked out me and the jeep will have a better understanding of each other. Good thing I have a refrigerator in the garage...

 

[jfrabat]

My 2.5L has always behaved more or less the same (although never reaching 1,500 RPM, but typically goes up to about 1,000~1,100 when coming off the freeway, and then drops down to normal... Never though much about it. But it could well be a vacuum leak, so I would check for that (then again, I have replaced all the vacuum hoses on mine for that same reason and still does the same!)