Last year I had this problem, and now this morning. My check engine light comes on an it says it has to do with a high TPS. Last time I just squirted the tps with WD40 and actuated it a few times by hand, and that fixed it. I hope thats all I have to do this afternoon, it is too cold to do anything.

 

I measured my TPS. 1v closed and 4.25 open. My chilton manual says aprox. 1v open and aprox 5v open.
I don't know I would say close enough but maybe the experts have some comments.

 

Its out of spec. Is that the issue? Not sure but if it was my jeep and given the symptoms I'd be replacing it.

 

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.

 

No one has ever seen the IAC motor valve seat wear out? I can't get a new IAC motor to bring the idle down but the old IAC motor works??

 

Umm will test this on my 1990 xj

 

Mine does it lik every other day some time fine for a week but iam glad to find a way to check my sensors ??

if it a intake gasket how hard is it to replace
I am use to working on v8s not inline 6 lol

 

I'm finding out that this wasn't the magic fix, but it is working much better.
Sometimes when it's really cold the rpm will still come up to 1500 for a while.

 

So I guess no one has a clue as to why the old IAC idle motor so much better and the brand new out of the box IAC motor does not work properly?

 

just wanted to point out that I beleive the wires the Haynes manual lists to test are incorrect.(as to what wires have what voltages) I had issues with mine and a friend of mine looked it up on his manual subscription at his job and confirmed this.

Also I had some issues with the TPS the voltages tested fine so i cleaned it and the throttle body seemed to be better

 

Any ideas as to what the best thing to try first would be with this problem? I have a 96 that has started doing this recently. It's done it only about 3 or 4 times now. It ONLY happens after I make long trips at higher revs. I drove to St. Louis (about a two hour drive) and whenever I'd get off the interstate, it would hold a high rev in park and when in drive at a complete stop. The first couple times it would hold at about 1,000 rpm and a little higher, and would be perfectly fine if I just shut it off and started it right back up, idle would be back to 800 rpm. But last night I drove to St. Louis and back and when in park, would idle up to 2,000 rpm. I let it sit and rest all night (which used to do the trick) but started it this morning and it went right back to 2,000, then slowly went down to about 1,500. I know I have a crack in my intake manifold, but it's not too terrible, could this possibly be screwing with the amount of air pressure running through? Or should I check my TPS or clean my IAC? Haven't been able to check my vacuum lines yet, but I don't think they're the problem.

Any help would be GREATLY appreciated. 1996 Cherokee Classic, IL 6 (of course) 239,800 miles on her. Thanks everyone!

 

As well as checking the intake manifold for vacuum leaks, check the throttle body base gasket. If the bolts are loose, it could be leaking there. It seems the vacuum leak is temperature sensitive, that is when the parts expand, the leak closes up, thus slowing your idle. The neatest trick I saw to find vacuum leaks involved a propane torch. Don't light it, just play the gas around the gaskets. The engine will speed up when the gas gets sucked in. You don't need a lot of gas either. Just a little bit will help pinpoint the leak. The nicest thing about this method is it's dry and doesn't soak your engine with WD40. Quess what? WD40's propellant is propane and iso-butane. Both are flammable gases.

Rueter, if your intake manifold is cracked, I can't imagine it NOT leaking vacuum! Vacuum is the hardest thing to seal. Even the slightest gap will suck in air and change the idle speed. You can try to seal it with RTV but I doubt it will last.

 

I was planning to see if patching the crack would help at all for now. I don't really want to replace the whole manifold if it's not what's causing the problem. I'm in the midst of trying to get this Cherokee sold and would rather just patch it to get me through until it's gone. I don't want to drop too much more money into it just before getting rid of it.

 

Yeah, I can understand that and changing the manifold is a PITA! Top bolts are a piece of cake, bottom are the problem.

 

So would you say that the crack is more than likely what's causing this? Because up until last night, the ONLY time I'd have it idle at high revs like that is when coming off of long trips of maintaining about 75 mph and 2,000 rpm for long periods. It's my daily and until today have not had it idle high on me at all with daily commuting.