Mechanics

INTERMITTENT MISFIRE

1999 Jeep Cherokee

Problem: My 1999 jeep cherokee sport (94,000 mi, 4.0L - 6) starts fine every time. After 15-20 seconds it starts missing. If you put it under load it " backfires through the carb" (at least that's what it sounds like). It misses for about 15-20 seconds. Then the problem goes away. It's the same every time it happens. There is never a check engine light. Doesn't seem to matter if engine is warm or cold. Sometimes it will do it after only a two minute stop. But it doesn't do it every time.

My dealer said they saw the problem but could not find what was causing it, since it goes away so quickly. They said they would rather I drove it and when it got worse, they'd try to fix it. They said the only other option was to " easter egg" it. They would keep replacing parts until it got fixed. Well, it has now been doing it for about a year. No better. No worse.

Could use some help. At the very least, if I start replacing parts myself, which ones and in what order should I replace them.
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Asanctuary1961
June 3, 2007.



There are two services I would recommend on this one. First is a fuel injector cleaning, second is a intake valve cleaning. There is a product called seafoam that works good on the intake valves. GM top engine cleaner and caslte also make good products I have used.
The fuel injection cleaning should be done with a cleaner that involves disconnecting power to the fuel pump and running the motor off of a chemical cleaner. Another method is with a motorvac machine.

I also have a 1999 Jeep Cherokee Sport, 145,000 miles with a 4.0 6 cylinder engine. My symptoms are the same as you describe. I had a full tune up with the exception of a fuel filter. I'm thinking sensors but the computer doesn't throw any codes. Runs very strong otherwise.

Tiny
Walleye8s
Jul 6, 2007.
I have a 1999 jeep grand cherokee 6cyl 4.0l auto trans 2wd with 80k miles I am getting an ignition coil code, misfire on 3rd and 4th cylinder and catalytic converter too rich. I replaced the coil rail (distributorless ignition), spark plugs, throtle position sensor and finally the crank position sensor. Afetr about 10-20 minutes of driving the RPM fluctuates and the idle revs a few times and then it starts to shake like the timing it off. There are no leaks in the air lines or fuel lines. It stalls sometime when the engine shakes. I am all out of ideas and really don't want to pay a mechanic if I can do it myself.

Tiny
Decarolus
Aug 30, 2007.
Two things,
First start a new post.
Just cut, copy paste.
Second list the code number on the post.

I think that I may have had the same problem. See my post: 1998 Jeep Surging and backfiring

I replaced a the fuse in the O2 sensor heater circuit. When the O2 sensor is not a tempature it does not read correctly. The heater keeps it at the right temperature. If the engine is under load the sensor warms up from the exhaust stream. When cold or under no load the exhaust is not hot enough to get the sensor hot.

Fixed mine!

Tiny
Yeloduster
Sep 17, 2007.
Jeep has a tsb on this.

Fuel injector #3 receives too much heat from the exhaust manifold and causes that injector to " act up". Jeep has designed a heat shield that wraps around the injector to insulate it.

Tiny
Lostinmars
Sep 21, 2007.