Rough Idle, Loss of Power --> My fix -
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post #1 of 8 Old 11-18-2005, 11:22 PM Thread Starter
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Rough Idle, Loss of Power --> My fix

I'm a new Jeep owner and wanted to post my experience with rough idle & loss of power. It took me a couple of weeks to solve the problem, but with the help of this forum & and the search function, the problem has been eliminated. Hopefully I can repay the forum by helping others with search hits on:
- Idle Air Control Valve (IAC)
- Pressure Sensor (MAP)
- Throttle Position Sensor (TPS)
- Fuel Pump & Fuel Filter
- Battery & Alternator
- Oxygen Sensor (O2 Sensor)

I bought a Chrysler 300 SRT8 in May and recently made the decision to garage it for the winter. So I picked up a 97 I6 for winter use about a month ago. The body & interior were in good shape, but it had some issues. The T-case was leaking, so I got that fixed up. What was more insideous was the rough idle I began experiencing shortly after bringing it home. At first, I would describe it as a hunting idle. It would oscillate from under 400 rpm to 1K rpm. At the worst of times it would hit 200 rpm and almost stall before kicking back up. Interestingly, it didn't do this when I would first start it up in the morning (this turned out to be an important clue). When it was cold, it was rock steady @ 1K rpm.
I didn't want to take a 8 year old Jeep to the stealership for diagnosis and subsequent raping, so after asking around, I replaced the obvious choice, the IAC Valve. This was a waste of $100 as it didn't change a thing.
So I continued my search. When trying to find a pattern to it's behaviour, I made an observation: if I turned off all the power accessories (lights, fan, radio) sometimes the idle would stabilize. Conversely, if I turned on all the accessories (rear defog, fan on high, etc.) I could repeatedly put it into "hunt" mode. So I wrongly concluded the battery &/or alternator must be weak and screwing up the PCM or something. Well, I now have a new battery & alternator (probably not a bad thing since they both looked like original equipment), but still had a rough idle.
At this point, I discovered the forum and learned about fuel pumps, MAPs & TPSs as possible root causes. Fortunately, these I could test without replacing (especially the fuel pump!). I hooked up a pressure guage to the fuel rail and it was rock steady at 48 PSI (even during the hunting) and so I thankfully ruled out the pump/filter. I back probed the TPS & MAP, and the voltages varied as expected, so they seemed ok.
At a temporary impass, I reverted to a shotgun approach, and gave it a tune up. Again, probably not a bad thing to do, but it didn't help.
After a brief respite, I renewed my forum searches, which yeilded some fresh fruit. It was reported that a failure of the upstream O2 sensor could result in a rough idle. Further, it could cause power loss during initial acceleration. It was reported that you could "push through" the power loss, and the symptons would not return until your next session behind the wheel. I realized that I was so focused on my hunting idle that I hadn't acknowledged the importance of these symptons which I was also experiencing. The clincher was when I read that the upstream O2 sensor only functions when the car is in "closed loop mode". i.e. The PCM ignores the O2 sensor and swags the fuel mix until the car is warmed up. That would explain the steady idle on cold mornings. As an added bonus, I read that the O2 sensor was very sensitive to power/voltage levels. Hmmm. I was off to buy an O2 sensor.

That was the trick. She runs great now. Steady idle under all conditions. No power loss. I'm even watching my milage to see if there's any changes (too early to say).

In hindsight, it all makes sense now. The PCM must have been acting schizophrenic. The O2 sensor telling it to change the air/fuel mixture (which gagged the engine) and the CPS telling it the idle was too low (which it periodically corrected by opening up the IAC) --> hence the oscillating idle.

Any reservations I had about my purchase are gone now, and I'm looking forward to many fun miles enjoying winter in my Jeep in the GWN.


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post #2 of 8 Old 11-18-2005, 11:46 PM
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Welcome, and great post. There is a ton of good info here, and it's great too see it go to use.

There are also a ton of very bad jokes, some odd discussions, and really random comments, which makes a post like this even MORE worthwhile.

- Bill (formerly VetteDriver)
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post #3 of 8 Old 11-19-2005, 05:20 AM
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Hey welcome to JF

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post #4 of 8 Old 11-19-2005, 06:45 AM
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Great job and I am sure that this will aid others in their quest for smooth idling.

Cheers Steve

94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new
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post #5 of 8 Old 11-19-2005, 06:52 AM
Phil Gerard
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It's nice to see someone smartly walk though the information here and solve their problem!
Welcome to JF!!!


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post #6 of 8 Old 09-01-2007, 07:49 PM
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great post, Thank you! I have the same exact issue w/ my 96 gc ltd. 5.2l appreciate your eliminations. one thing though i still have a ghost in my gauges all stopping for a few min. while running ???
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post #7 of 8 Old 09-02-2007, 12:30 PM
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Good post/info for others!

96 Grand Cherokee Limited 5.2L
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post #8 of 8 Old 09-03-2007, 05:57 AM
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Funny, the original Poster made one and one only post since 2005. And it was an excellent one at that. But then he drove off the face of the earth?

Cheers Steve

94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new

88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new
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