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Old 07-15-2013, 03:11 PM   #1
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Please Help! 1997, 4.0, Sport Problem

Hi Folks,

Thanks in advance for any input.

I have a 97 TJ sport 4.0, 4 speed automatic with 193K miles on the clock.

I have had the car looked at by two shops and neither were able to help with the problem I'm experiencing, I'll try explain the problem with the most consistent set of circumstances, because it has been pretty random.

Car will sit for a day or two, within about 5-10 minutes of driving the car (speed seems unimportant, has happened at 30 or 60mph) and then i'll lose acceleration, and the engine will sound like it's going to conk out.. engine revs and then drops revs and then drops etc. After about 2 minutes the handful of times this has happened everything has been a ok... and driveability has been ok...

No engine codes.. best guess so far is fuel pump? is this plausible.. I'm not a mechanic, I have a basic knowledge of things..

02 sensor/cat/tps etc was ruled out apparently because it should kick out engine codes, and shop got nothing... hardest thing of all is Jeepy has not repeated the problem while being driven by a mechanic..

Any help is appreciated... thanks.

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Old 07-15-2013, 04:42 PM   #2
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Sounds like you might have a vacuum leak somewhere... take your ccv off the orifice on top of your valve cover... blow cigar smoke in it... where the smoke comes out is your leak. I think it's a vacuum leak because you say it happens when cold... not hot.

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Old 07-15-2013, 09:36 PM   #3
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Thanks very much for this input.. I'll look into this asap.
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Old 07-15-2013, 09:48 PM   #4
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A dead spot on your TPS or a broken honeycomb in your cat could do that without throwing codes. Next time it does it drop into 2nd or 3rd gear and maintain light throttle to try to rule out the transmission slipping between gears or the TC locking/unlocking rapidly. Then smoothly and slowly run the accelerator pedal through most of its range (assuming it's a safe area to do so). If it always bogs down at the same place (pedal distance, not RPM/speed) it might be a dead spot on your TPS telling the ECU that you are at idle or WOT.

Lastly crawl underneath and give your cat a good whack with thick leather glove on, or lighter whack with a rubber mallet. If you hear rattling there may be a loose piece of substrate floating around in there. If it's just the right size it might occasionally block the exhaust and choke the engine.
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Old 07-25-2013, 09:15 AM   #5
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Checked for vacuum leak.. didn't appear to be the problem.

When it rains the problem is MORE regular..

When the car has sat for a day or two it's ALSO MORE regular.

I'll check for the CAT and TPS suggestions made.. Thanks very much.
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Old 07-25-2013, 09:22 AM   #6
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Don't rule out the o2 sensor/cat just because it hasn't thrown a code. A bad o2 sensor can be spotty and still not turn the CEL on. My 4.0 ran like sh*t with no CEL and replacing the front o2 sensor made it run much better.
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Old 07-25-2013, 09:55 AM   #7
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Quote:
Originally Posted by Robertsk1 View Post
Don't rule out the o2 sensor/cat just because it hasn't thrown a code. A bad o2 sensor can be spotty and still not turn the CEL on. My 4.0 ran like sh*t with no CEL and replacing the front o2 sensor made it run much better.
X2, the engine computer on earlier TJs has no reliable way of telling if an 02 sensor is giving bad information or not. Newer TJs have more 02 sensors & more sophisticated monitoring/algorithms that are more apt to generate a CEL with a bad 02 sensor but not so on early model TJs.

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