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Unread 11-23-2003, 06:02 PM   #1
dmask0
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1977 CJ5 
 
Join Date: Nov 2003
Posts: 67
Service Engine Troubleshooting Help

I was looking for some advice/help with solving some engine problems on my 1998 4.0 4x4 with 76,000 miles. I've had a real rough idle for the past year. Throughout the year, I've changed the plugs, wires, rotor & dist. cap, and had a professional fuel system cleaning. None of these ever solved the rough idle for even a day. Also, the day after installing the new plugs, I pulled them to double check the gap spacing and noticed they were wet with gas. I finally got a 'service engine' light this past weekend, and the code was 0301 for 'cylinder 1 misfire'. I know I'm running a rich mixture, but can't find the source. I've traced the vacuum lines as best I could, and I pulled the Idle Air Control Valve and cleaned it thoroughly with brake cleaner. The rough idle and hesitation can only be felt at idle at stop lights and doesn't seem to be dependent upon weather temperature or humidity. There is no EGR vavle on the '98 to check. Does anyone have any advice for troubleshooting or has anyone had anything similar in the past? I checked the old postings on the "rough idle" topic, but there didn't seem to be any solutions posted.

Thanks in advance.

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Unread 11-24-2003, 08:50 AM   #2
Intrepidjeep
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Try getting the engine error codes

Quite a few of the possible culprits are pretty cheap. For example, a throttle position sensor (TPS) is around $40 or so. The TPS can very easily cause this problem. In other words, you could try throwing easy-to-replace parts at it up to the point that you just need to take it to a shop for diagnostic....

When you do this you might consider disconnecting the battery for a while to let the battery clear all its codes and re-learn how to run the engine when you restart it. Of course while it's relearning, it may run rough for a little while as the computer does some fine-tuning....
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Unread 11-24-2003, 02:05 PM   #3
slaterny
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re: the Throttle Position Sensor --

I bought a new one to swap out the old, and it is not screwed in with regular screws...appears to be some type of star shape...! It has been impossible to get in there to unscrew (placement doesn't help either)..hate to have to bring it to the mechanic because I can't unscrew a screw!

When I startup in the a.m., it sometimes revs up to 2000rpms, and I have to keep shutting the engine and restarting...after about 5-6 times, it drops to normal idle...

I know it's the tps because when I unhook the electrical connector from the tps, the idle drops down to normal, and when I put it back, it revs high again..

Why couldn't jeep just used regular phillips head screws?!?!?
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Unread 11-24-2003, 02:49 PM   #4
Intrepidjeep
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That sounds like classic TPS failure. If my fuzzy memory serves, the TPS should provide a voltage range of somewhere around 1.5V (throttle closed) to 3.7V (throttle open). (Of course those numbers may be off a bit but you get the idea.)

If it's providing the wrong voltages then the computer gets confused and doesn't provide the correct air/fuel mixture.

In the shop manual (possibly also Hanes/Chiltons?) there is a very easy test with a voltmeter for checking the TPS. If the voltage reading you get is wrong, then the TPS is bad.

The non-philips screws are probably a lame (see below) effort to discourage backyard mechanics from doing it themselves.

I haven't looked at my own TPS (though I think I plan to buy one to replace it shortly) but I suspect they're Torx screws. If so, you can pick up a set of Torx screwdrivers most anywhere that sells hand tools (Sears, etc.).

In my GC, the engine exhibits heavy pinging on 87oct fuel, and noticeable pinging with 89oct fuel -- I'd have to mix in about 10 gallons of premium to get the pinging to stop. Assuming I don't have heavy carbon buildup (which I doubt since I always use Chevron and have tried various carbon-cleaning products to no avail), I'm thinking the TPS may be giving wrong readings, making the computer provide a leaner mixture (too little fuel). In your case, sounds like the computer is providing a richer mixture (too much fuel).
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Last edited by Intrepidjeep; 11-24-2003 at 03:01 PM..
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Unread 11-25-2003, 12:12 AM   #5
dmask0
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I'm going to test the coil, the TPS sensor and my spark plug wires tomorrow. I've found some good info on a chilton's website on how to do this. Thought I'd pass it on to everyone.

http://www.cybrrpartspro.com/Manual%...delLookup.html


also, i recently cleaned my Idle Air Control Valve. To remove it required a #20 star drive and a #20 safety star drive. I am sure these are the same star pieces previously mentioned. I picked these up in a screw driver kit at the local part store for $10. But be careful with your first couple turns, as they are easy to strip. I actually loosened all four with some vice grips first, then proceeded with the star drives for removal.
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