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Discussion Starter · #1 ·
I'm a relatively new jeep owner. I have a 1999 TJ - L6 4.0 . I'm having a problem with my jeep randomly shuting down. I will be driving and all of the engine shuts down. Sometimes I can restart it in neut while I'm still rolling. Other times it takes 1/2 to an hour of sitting before it restarts. I've already replaced the coil, cap, rotor, sparks wires, plugs.. Still getting the problem. No one seems to know exactly why it's doing it.


Very frustrated :chair:
 

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Could be a sensor, gone bad. It could be a clogged fuel filter. It is hard to say, without doing some basic, step by step, diagnostics.
 

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Well if the ignition is just cuting out I would think it would be a bad connection some where.:)Does the check engine light come on?
 

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Discussion Starter · #6 ·
Could be a sensor, gone bad. It could be a clogged fuel filter. It is hard to say, without doing some basic, step by step, diagnostics.

Thought about bad fuel filter/reg. However there is fuel under pressure in the fuel rail. (depressed the schrader valve and fuel jetted out). Swapped around relays btwn horn and fuel no diff... When it's "out" the ign coil is not outputing anything. Swapped the ecc relay when it was out and no difference.
 

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Discussion Starter · #7 ·
Well if the ignition is just cuting out I would think it would be a bad connection some where.:)Does the check engine light come on?
Couldn't find one... Had about an hour last time before it restarted, checked over all my connections couldn't find one. I don't believe I seen a check engine light on.
 

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Do you have a dealership, in your area? I would ask their service department, what they think. Some of them will actually talk to you on the phone, without the "You have to bring it in here, for us to diagnose it". Just a thought. I know there are idle sensors, and few other sensors, that could contribute to this problem.
 

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Obviously this is just a shot in the dark, since I cannot be there to diagnos the Jeep, so please don't run out buying parts.

When I was in school, during Drivability Diagnosis class, our teacher told us about this same sort of scenario. He said it happens to EVERY tech sometime in there life, and it will take days to figure it out.

Driving down the highway, been driving about 30 minutes, its a nice hot day outside and BAM all the sudden the motor shuts off, and all tests are good. Once the car sits for awhile, and cools down, and starts right up and drives no problem. No codes found stored in the computer. The car will do this intermittantly, or happen frequently. Warm up, cut off, cool down and start right up. Usually surfaces during hot summer months (July-Sept) not during winter.

He said the problem is 90% of the time going to be the PCM. Yes, the main computer. He said what happens is (and it makes sence with a computer background) the computer is fine when cold, but being old and whatnot, the computer is worn down. When it gets hot, a solder point losses connection, or a resistor goes bad or something like that, I forget exactly what it was, but it goes out with heat, then shrinks back to normal and makes a solid connection when cold. He said its actually pretty common.


Now with that said, I worked on cars for 4 years, and then did Restoration work for over a year, and only had to replace a computer once. Don't go out buying a new one, but if you have a buddy that wouldnt mind switching PCMs with you for 2 hours or so to see if it fixes the problem, it wouldnt hurt.
 

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Discussion Starter · #10 ·
Sounds exactly like what happens. Just took it up to autozone to run on the computer diag. No codes found. haven't had the trouble that long, so I couldn't tell you if it happens in winter or not... but thanks... had another suggestion that it was that.
 

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like I said, its just a shot in the dark, and I wouldnt spend hundreds/thousands on a new computer just because I said so, BUT what my teacher described to me a few years ago seems very similar. Just something to chek since you have fuel/air and spark is basically all good unless you got a bad part or your PCM is going out.

Find a buddy with a Jeep similiar to yours, IE Same motor and trans, close to the same year, and swap computers. Just for 2 or 3 hours. Drive it around town and on the highway, getting it warm, doing what you do when it clunks out, and see if it fails. If you dont got a budd with a Jeep, look up a Jeep club in your area.
 

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Discussion Starter · #12 ·
New thing I noticed... actually by accident. I left my A/c on as I turned off the jeep ( was running atm) and the blower still stayed on even after I had the key out of the ignition. Normally I shut down the a/c before removing the key so I didn't notice it. Also it was sugested that it may be vapor lock in the fuel line? Is that possible with 1/2 tank?
 

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JCS05Rubi said:
Obviously this is just a shot in the dark, since I cannot be there to diagnos the Jeep, so please don't run out buying parts.

When I was in school, during Drivability Diagnosis class, our teacher told us about this same sort of scenario. He said it happens to EVERY tech sometime in there life, and it will take days to figure it out.

Driving down the highway, been driving about 30 minutes, its a nice hot day outside and BAM all the sudden the motor shuts off, and all tests are good. Once the car sits for awhile, and cools down, and starts right up and drives no problem. No codes found stored in the computer. The car will do this intermittantly, or happen frequently. Warm up, cut off, cool down and start right up. Usually surfaces during hot summer months (July-Sept) not during winter.

He said the problem is 90% of the time going to be the PCM. Yes, the main computer. He said what happens is (and it makes sence with a computer background) the computer is fine when cold, but being old and whatnot, the computer is worn down. When it gets hot, a solder point losses connection, or a resistor goes bad or something like that, I forget exactly what it was, but it goes out with heat, then shrinks back to normal and makes a solid connection when cold. He said its actually pretty common.

Now with that said, I worked on cars for 4 years, and then did Restoration work for over a year, and only had to replace a computer once. Don't go out buying a new one, but if you have a buddy that wouldnt mind switching PCMs with you for 2 hours or so to see if it fixes the problem, it wouldnt hurt.
As a mechanic/engineer I agree. If you have ever heard of the red ring of death on an xbox it's basiclly the same thing it gets to hot and the soders break because the silicon electronics board will warp. Often if a PCM is bad it for a reason so I would check for shorts or anything that could heat up the PCM like hot air. Bring the most expensive electrical part on your jeep I would take this guys advice on finding a friend just make sure its the same PCM otherwise you could damage your buddies.
 
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