Oscilating idle and stumbling engine

[rendl]

Here comes the magic:

It starts with the idle fluctuating. It is like an oscillation between 500 and 1500 rpm or sometimes between what looks like between 300 and 1200 rpm. It is an oscillation of about 1Herz, if you know what I mean.
It never does it when the engine is cold. May come after 30 Minutes, may come after 4 hours of driving.
If it were only that occasionally, I wouldn't mind.

However when it starts to do that, sometimes the engine starts to stumble. It sounds like it is fuel starving. Sure in that case there's almost no power and it smells.
That kind of stumbling also happened after a couple of hours pulling the tent trailer, so at 60 mph. However the engine never completely stalled. Well when you're just 5 miles close to the campground, what do you do? Switch the Overdrive off and push it into a lower gear..

We have replaced all kinds of sensors. I have a great dealership, but they couldn't find it out yet. I really spent a lot of money and I would really like to find out what the problem is but never did the car have the problem during its stay at the dealership :-(


The dealer has checked all vacuum hoses. The fuel pump pressure on the rail has been checked as well. They even let the Jeep running for an hour or two with an osciloscope connected. I understood they monitored the Amps on the fuel pump. Unfortunately the problem did not show up. They made a couple of test rides, but everything was fine. Sure enough after a couple of days, when I drove the car (or when my wife did) the problem showed up again. Finally I really thought it would be the Crankshaft position sensor or the fuel pump. After the dealer replaced the CPS it looked good for a week, then we had the oscillation and the stumbling again. It was 15 miles after we had filled up the gas.
That was one week ago.
Dirt in the tank maybe? It is so hard to determine since the problem only shows up every week or so. Right now it is fine but it's starting to get annoying if you can't trust your Jeep :-((
I do not want to waste again money for replacing the fuel pump, just to find out the problem persists.
Are we searching in the wrong area, could this be an ignition thing after all?
PS. There is no error code.

Every idea will be really appreciated!
THANKS.


Grand Cherokee LTD 1995 V8 5.2 89K miles
Recent works & Replacements:



- Repl. Throttle position Sensor TPS
- Repl. Idle Air Control valve IAC
- Repl. Oxygen Sensor
- Repl. Fuel filter
- Repl. Crankshaft position sensor CPS

- Checked hoses
- Monitored fuel pump

- Repl. AC piping and condenser dryer
- Repl. Catalytic pot and manifold

[xfsrg]

crazy enough. i had a very similar problem - i hesitate to say same exact thing but sounds like it. my engine would sort of "stall" then start going again. I took it in for a tune-up and they serviced it and they forgot to hook-up the engine temp sensor. so it worked right for a day/two then acted up again. Finally, after a summer of having this problem, i took it in and had them reconnect the engine temp. Amazingly, this fixed the problem.

im sorry i cannot be more specific about what the problem was, but it seems that when they fixed the engine temp sensor the problem has not occured anymore.

[rendl]

Originally Posted by xfsrg

crazy enough. i had a very similar problem - i hesitate to say same exact thing but sounds like it. my engine would sort of "stall" then start going again. I took it in for a tune-up and they serviced it and they forgot to hook-up the engine temp sensor. so it worked right for a day/two then acted up again. Finally, after a summer of having this problem, i took it in and had them reconnect the engine temp. Amazingly, this fixed the problem.

im sorry i cannot be more specific about what the problem was, but it seems that when they fixed the engine temp sensor the problem has not occured anymore.

Thanks for your ideas!
My temp sensor always works fine, so I guess that's not it.
By the way it happened again to day. Again 20 miles after I had filled up the gas!
As usual the idle RPM dropped and started to oscillate.
It did it acouple of times and when we came back from our trip at about half gas tank, it did it again. This time it even brought the check engine light on. There also was a code 21. I've got to check what this code referrs to. I think it is supposed to be O2 sensor. Well we just had replaced this one.. Let's see; I'll report back.
If anybody has a different idea.. be my guest! :-)

[Cam7]

Code #21 =oxygen sensor input voltage higher than normal. I have to look into code and post back.

[msrjeep]

what brand of O2 sensor? i have seen problems with bosch and other aftermarket when used on chrysler vehicles. may have to use oem mopar..

[rendl]

It is a Bosch one. The guy at the parts shop told me it qualifies as the original replacement. I do not believe into a possible O2 Sensor problem anyways, even if the error code is set. I rather tent to believe the O2 error is a kind of a "side effect" of the engine terribly stumbling...

Thanks for your feed back about the O2 sensor.
You said you had probs with the Bosch ones. Could you please detail what kind of issues?

As far as I know if the O2 information is bad, shouldn't the engine management fall back to a standard program at least? Would the stumbling make sense to you in that case?

[dksob81]

well it could be the O2 Sensor. for ex. My father in law has a 98 Olds Achieva, his O2 sensor(s) is bad causing his engine to run lean. but he is stubborn and stupid and won't buy the parts until the vehicle won't drive anymore, he always waits until the last moment to do anything. well anyways, the car gets terrible gas milage, the CHECK ENGINE light is always on, and when ever u give it gas it stumbles and has no power at all. and this especially sucks for going up hills.

[ChadEbert]

gee, dksob, all those things you mentioned are standard features on the Achieva. <laughs>

[rendl]

Good news! Believe the problem is solved.

When my dealership checked the Jeep last time after I had a code 25, they did not find anything wrong with the wiring of the (new) O2 sensor.
However they discovered that the cooler reservoir had spilled water onto the engine management module. They found moisture inside the module. They dried it and sealed it.
Now it has been almost 4 weeks without any problem and I consider it being solved. No more ocillating idle and stumbling engine anymore !!!Knock on wood..

Thanks everybody for the help!
:-)

[rendl]

Sorry. Got to give it a smiley!!

:-)