Jeep Grand Cherokee has fast idle cold then stalls when warm.

[jeepjerk]

1993 Jeep Grand Cherokee 4.0L straight 6 with 200k miles.
Just replaced plugs, wires, air filter, IAC motor, oxygen sensor.

High rpm after startup. Doesn't want to go down. Then wants to really dig in and stall when put into drive. Doesn't do this when putting into reverse????

Is this an engine problem?? or is this a transmission problem??? or both???


I think I have gone through about 4 IAC motors and 3 oxygen sensors in this jeeps lifetime fighting this idle problem.

But this seems different in a way. At a cold start the idle will go way up (2K)
Sometimes engaging the AC will bring it down.
Then after it warms up the idle goes really low (500) and the engine wants to stall when put into drive.
Now what is really strange is that when idling low it will go into reverse and not threaten to stall???

Now I just replaced the IAC motor and it adjusted the idle fine after a computer reset (pull neg bat terminal).
Now back to high idle cold and warm stalling in drive.

I read the codes and found oxygen sensor fail code. So changed the o2 sensor and reset.

Still the above symptoms.

At this point I am at a loss.

Thanks for any help.

[fredjacksonsan]

What year? What engine? How many miles? How long has it been since tuned up?

[jeepjerk]

I don't know what you mean by tune up??

I've replace just about everything I can think of.

The computer controls timing.

No error codes after replacing o2 sensor.

????????

[jeepjerk]

Well I guess this is far too common a set of problems to bother answering.

I think the only things I haven't changed are map, and crank sensors.

If someone could steer me to some big threads here dealing with this problem that would be great.

[fredjacksonsan]

Originally Posted by jeepjerk

I don't know what you mean by tune up??

I've replace just about everything I can think of.


????????

You didn't say if you'd replaced the plug wires, or the cap and rotor (part of tune up). You've already done everything else that I'd think of to do.

As far as the lack of responses, not everyone is on every day.

[jeepjerk]

Yes I've replaced the wires and cap within the last year. I checked the cap and it looks fine.

There is something here that I'm missing.

I started the car this morning and the rpm was about 1500 and was slowly creeping up to 2000.
I put it in reverse to back out of the garage and it worked fine.

Then put it in drive and from 2000 it was pulled down to about 300 and seemed like it was going to stall.

Is there actually a pvc valve in these cars? All I see is a hose going from valve cover to filter box.

[fredjacksonsan]

I'm not sure on the pcv.

One other thing I can think of....after re-reading your original post, you've got 200k miles on. That's a good run for the best of auto transmissions. It might be time for a rebuild. It seems that all the "usual" sensors and parts have been replaced.

I'm reminded of a time I went to test drive a car with my brother. It bogged down at a standstill, but once you gave it gas it accelerated. After driving and going through several shift cycles, we figured out that it was in both 1st and 2nd gear when not moving...

[jeepjerk]

I will admit I don't know a whole lot about automatic transmissions.

When you get on the road with this car there is no problem. Plenty of power and it shifts through the gears fine.

You are saying that the transmission can be in two gears at once?

So I manually put the car into first gear to start out. Same symptom.

How did you determine it was in first and second at the same time?

Anyway there is still the issue of idle being so high while cold and to low when warm.

[fredjacksonsan]

If the transmission works ok otherwise, then it could be fine.

I'm basically out of ideas, I'm not the sensor expert. Last thing I can think of is to see if there are any codes. You might want to swing by an auto parts store; many will hook up a scan tool and let you know what the results are.

[jeepjerk]

OK thanks for some ideas.

I know how to read the codes by turning the key 3 times and the fourth will give the codes.

At first there was an O2 sensor code and that went away with the new O2 sensor.

[fredjacksonsan]

I think that your year has the OBD-I, which doesn't show all the codes for transmission.

[jeepjerk]

ok thanks Fred.
Probably not.
Sounds like a trip to mr. badwrench is in order ;-(

[jeepjerk]

Today I checked the MAP, MAT, and ECT sensors. whew

Everything tests good here.

When I pull the vac line off the MAP sensor my high idle goes to normal but according to the voltages it's working correctly.

I checked every vac line for leaks and found none.

[tim_gruber]

My daughter's 1993 Jeep Cherokee, 6 CY, 4L, (225,000 miles) behaves the same way except it idles normally when cold but stalls when hot. If she drives it without stops and goes it runs like a champ.

A trustworthy mechanic worked with it for about a 1/2 day and he could not figure out the exact cause. Everything showed to be within specifications on the computer except he said it was indicating that the fuel/air mixture was too rich (he also said that's how the exhaust smelled to him). He theorized that the car was essentially flooding when sitting still and hot. He thought this could be indicative of a fuel injector that was getting stuck at least partially open.

We've replaced plugs, wires, rotors, cap, run Lukas fuel injector cleaner through it with no noticable change. If the engine is allowed to cool (typically overnight) it starts and runs fine the next day until it heats up again and she starts driving in stop and go traffic.

All sensors check out OK. No check engine light and no old codes present. Mechanic suggested putting new fuel injectors on it to eliminate the stuck open theory. My mechanic is an honest guy and he did not want to charge us his shop price for doing a job that he could not guarantee would fix the probmlem. He recommended that I but some injectors cheap off ebay and do it myself - which is my next project. I found a set of 6 for $60 so I figured what the heck.

Has anyone actually managed to find and fix this problem in an older Jeep Cherokee?