Oscillating RPM's Problem

00GTP4ME · 06-22-2009, 04:00 PM

Hello All!

I own a 2000 Jeep Grand Cherokee Limited 4.7L V8. I get strange oscillating RPM's whether my engine is cold or warm, and whether I'm idling or driving and it drives me absolutely crazy. Its seem that when I'm in reverse, it has the most drastic oscillations. I notice the problem most, however, when the engine is cold, and when I'm driving on the freeway with the cruise control on. The RPM's just go up and down, up and down. If I'm starting up the Jeep and happen shift in or out of reverse when the oscillation is at a down swing, the engine has occassionally died on me although that's rare.

I don't know if this is related or not, but I had the check engine light come on a few months ago with a Trouble Code: P0204 - Cylinder 4 Injector Circuit Malfunction. I called my mechanic and he suggested (before diving into any costly repairs) that I get some fuel injector cleaner, run it through my tank, and see what happens. So I did that and a day later the check engine light went off, not to come back on now for a few months.
I also noticed when I bought it that the engine absolutely insists on operating at 210 degrees, no matter what. I knew 195 was the stat rating, so I flushed the entire system, replaced the t-stat, and nothing changed. I've checked the coolant level and it has stayed clean, and hasn't dropped a bit (no leaks). I was speaking to my brother the other day who is a back-yard mechanic and he suggested that the coolant temp sensor might be on its way out.
I don't know if this is anyway related, but there's some history.

I've owned the Jeep for a year now (my first Jeep) and when I've had it in for maintenance, I've told the mechanic to look out for the oscillating RPM's thing and they haven't been able to see anything unusual (of course!) It is driving me absolutely insane and I was wondering if anyone else has had a situation more similar to mine and what the solution was. Sorry for the winded story! I really appreciate anyone's time, suggestions, and help!

Thanks a lot.

Bob D. · 06-22-2009, 11:21 PM

Quote:

I also noticed when I bought it that the engine absolutely insists on operating at 210 degrees, no matter what.

The reason it insists on running at 210 degrees is because that's where it's designed to run, 210 is normal operating temperature.

As for the rpm oscillations, it's either the IAC, the TPS, or a vacuum leak.

nova73noir · 06-23-2009, 07:44 AM

i already have this problem it was 02 sensor but check engine ligth was on!!! inspect your ignition coil for crack its maybe a humidity problem? hope that's help!

00GTP4ME · 06-23-2009, 09:09 AM

Last night I decided to try replacing the TPS. I disconnected the battery, took everything off until I got to the throttle plate and I made sure to clean that off really good with some carb spray (it was kinda gummed up). I put the new TPS on, reconnected the battery and boy, did it start up rough that first time

. After the first start though, it really seemed to run much smoother. The idle wasn't near as high as it usually seems too. Usually it idles around 800-1K RPM's and if it drops below that, is when I start seeing the oscillating. I sat there and just watched it run and it seemed totally fine at around 550-600 RPM's. I did, however, hear the strangest clicking noises going on inside the dash until I took it for a good test drive and then they stopped. Just stuff resetting from my disconnecting the battery?

Anyway, took it on the freeway and put it on cruise and the RPM's didn't move. Got off the freeway, drove it around town a little bit, same story. If it keeps driving like it did last night, I'd say this one is licked. It has had it's good running spells before though, so before I go celebrating, I'm going to give it a couple days to make sure it's not a fluke.

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