High Idle when started cold or warm

[jeepjerk]

High idle when started, cold or warm 2k rpm. The idle will come down when thoroughly warmed up to about 1k rpm, but seems to lug when put into drive.
I did a pretty thorough search o this subject but didn't find as many threads as I would have thought
1993 jeep grand cherokee LT 4wd, 4.0ltr 6cyl engine
Replaced spark plugs wires and cap with no change.
I'm on the third new oxygen sensor.
Just installed the fifth new IAC motor. When I installed the motor I reset the computer.
The new motor caused a very high idle 3k rpm. Then the idle came down in steps to about 800 rpm like it should. Seems like it isn't adjusting anymore. Sometimes when I engage the air conditioner the IAC will compensate for the additional load.

I can't find a vacuum leak anywhere. I checked the MAP sensor and get the correct voltages for vacuum and normal air pressure. Sometimes when I remove the vacuum tube that goes to the MAP sensor the idle will come back down, but back up when connected.

I thoroughly cleaned the throttle body with carb cleaner. Didn't really change anything.

Like I said above the Idle will come back down when the engine is thoroughly warmed up, but it will go way up again when re-started while warm and takes a while to come back down again.

Well seems like all the old trick don't work anymore. I would be grateful for any tips and ideas to try.

Thanks

[fredjacksonsan]

It sounds like a bad sensor in the warmup circuit. IIRC there's a thread about it somewhere here, maybe in the regular Cherokee section. I'll try to locate it.

[fredjacksonsan]

Found some:

http://www.automotiveforums.com/vbul...hlight=warm+up

http://www.automotiveforums.com/vbul...hlight=warm+up

[jeepjerk]

Thanks Fred

This one will stay up around 2k rpm for 20 min or more depending on how cold the engine was. Then it will idle down to a little under 1k rpm.

Then shut off and immediately restart it will be back up to 2k rpm and just stay there
for a long time.

You may be right. It's like the PCM doesn't know it's already warm.

But I think it stays up in idle way to long even when cold.

We've gone through this BS with this car many times now. Like I said above we've gone through 5 IAC motors and 3 O2 sensors. All dealer fixes and the problem
resurfaces again very soon.

This is the same car with the tranny problem and I think this may have had something to do with it failing.

[fredjacksonsan]

Urg, tranny + engine is never fun. Sounds like the stealership is living up to its name....no way would 5 (or 3, depending on the part) parts be bad in a row!! There's something they are missing when they hook it up to the computer. They did hook it up, didn't they?

[fredjacksonsan]

Also, after poking around online some, it seems that it could be caused by a simple vacuum leak....so check all the vacuum hoses carefully.

[jeepjerk]

Hi Fred yes they did hook it up to their computer. This year is not OBD compatible unfortunately.
Yes I've been over the vacuum lines many times. But I guess it wouldn't hurt to check them again.
I'm feeling another trip to the dealer is in order. Doh!

[jeepjerk]

Just out of the blue the other day I decided to try one of the old IAC motors.

Amazing the idle doesn't race like it used to, even when re-starting warm.

Is it possible that the throat the IAC plunger sits in is worn to the point that it will
only work with a plunger that did the wearing?

I still don't think it's absolutely right but I won't argue with some success.

At least it's not racing at 2k rpm when started.

I guess the worn IAC throat could be called a vacuum leak.

[jeepjerk]

no one has ever seen a IAC motor valve seat wear out? Seems to work with old IAC but the new one can't get the idle down.

[jeepjerk]

I guess no one knows why my brand new IAC motor doesn't work and the old one at least tries to work???

[danielsatur]

Also check Similar threads at the bottom of yours, after Google Ads.

[jeepjerk]

Thanks I checked some of those links. Maybe some relevance but none of those for Jeep grand cherokee

[jeep2002]

tps will do that too