Maddeningly intermittent starting problem

[Seeker]

1043727

I have a 95 Cherokee Sport 4.0 High Output straight six with about 131,000 miles. It has been a great Jeep, but over the last 4 mos it has been very frustrating! Periodically it will refuse to start. Usually this happens after it has been driven for a while, but last week it just quit on me on the highway after about 15 miles. After it was towed back to my mechanic's shop (for the 5th time) they pushed it into the bay, he began by cleaning a few of the connections turned the key and it started right up. He has even kept it for several days several times to see if it will quit on him. Only one time has it quit on him so he could run diagnostics on it. In this case the computer tested bad (engine control module, I believe). At this time we replaced the computer. In addition to the computer being replaced, the fuel filter has been replaced (fuel pressure checks out ok), crank sensor replaced three times to make sure no defects (I was only charged for one), ignition coil was replaced, and the pick-up coil has been replaced two times. The last time it quit on the highway there was a sort of "whump" sound and it just quit. It always cranks like it wants to start, but sometimes just doesn't. Two times we thought we had the problem fixed. It ran for almost a month and the problem started again. Any help would be greatly appreciated! I really don't want to get rid of it![addsig]

 

[MudderChuck]

1043903

Change the fuel pump relay, cheap insurance. Take the cap off of the old one and look close at the contacts, may look pretty used up. Intermittant problems will drive a sane man crazy. When all else fails, have seen the EGR valve stick on occasion. Had a leaf, some kid, stuck in the fuel tank, that would float up and periodically close off the fuel inlet (do you have a locking gas cap?). Have on many occasions found loose connectors that sometimes make contact and sometimes don´t. My only real talent as a mechanic is stubborness. Have on occasion seen a bad ignition switch, poor contact. There is a bi-metal in the fuel pump that will shut it down for a few minutes, when it overheats. Usually only happens after driving for awhile. If it starts after a cool down period, this might be something to look at. I can´t really picture the fuel line routing for the 95, but have seen a metal fuel line, touching the exhaust manifold on occasion. Boils the fuel and causes a kind of vapor lock. Good luck.

[addsig]

 

[Seeker]

1043947

Thanks, MudderChuck, I'll check those out. I do have a locking gas cap so I feel pretty good that nothing like that got in. I'll change out the fuel pump relay just to be on the safe side, and I will definitely check out the fuel pump shut down as it seems to not want to start after its been run for a while and suddenly will once it has sat for a bit. I may also have the wiring harness replaced if that doesn't pan out, just to make sure there are no shorts, although when tested we are getting power everywhere. However, if it is a loose connection it may work sometimes and other times not. I'm with you on the stubborness. Not being able to figure this out just makes me more determined. I really don't want to admit I was outsmarted by my car!

[addsig]

 

[MudderChuck]

1043992

Changing the wiring harness is really labor intensive. But systematically checking all the plugs many times pans out. Each plug has to be unplugged systematically, inspected with a flashlight, for corrosion and bent or loose pins (I´ve seen the pins slide in the connector). And reseat the plugs completely. Even if you don´t find your problem, you will invariably find some loose connectors, corroded connectors or some with moisture in there. On earlier models many of the cables ran undernieth the carpeting and would often wear through, causeing parital grounding through the moisture under the carpet. Newer models seem to favor shielding the cables with plastic pipe. I don´t know if the 95 has the fuel pump cables in the frame or under the carpet. I´ve never given up yet, but sometimwes if I charged by the hour it would often be cheaper to burn the sucker. Might also take a close look at the fuel shut-off, the small tin can looking thing at the beginning of the injector rail, it´s vacumn operated on most models and has been known to cause problems due to low vacumn and such.[addsig]

 

[Seeker]

1044157

Hey, MudderChuck! You mentioned the fuel pump being bi-metal, do you know if they are still making them that way or has it been changed? I will definitely look into that as it always seems to not want to start after it has been run for a while. Thanks again, I'll keep you posted as things progress. I've got the matches out, but not yet ready to strike!

[addsig]

 

[MudderChuck]

1044363

Every fuel pump or for that every pump I´ve taken apart has a bi-metal in the windings. On numerous occasions I´ve replaced them to save the price of a new pump, It´s really not a job that was meant to be done though. You´re computer should show a trouble code, if the fuel pump is malfunctioning, even if it is intermittant, the codes don´t go away, until reset, on most computers. The bi-metal is like an auto reset circuit breaker, fairly primative, that is supposed to save the windings from melting down. It usually isn´t the problem, but indicates another problem like shorted windings or bad bearings causeing the pump motor to overheat. There is also a auto reset circuit breaker in the fuse box, to protect the fuse box from meltdown. On the older models the fuse box was to the left of the brake pedal and slightly higher, and was often kicked when entering and exiting the vehicle. Often the cover would fly off, and the relays, circuit breakers and all were hanging out there to be booted the next time. It´s worth a look. The next time you take the gas cap off, listen to see if it makes a sucking sound, I´ve know the vent holes (one way valve) on the gas cap to be plugged, causing the symptoms you have mentioned. Some newer models have no venting on the gas cap, have heard of a gas cap recall, but not sure if it applies to your year and model.[addsig]

 

[Seeker]

1044501

MudderChuck, thanks. Will check it out!

One of these days I'll get to the bottom of this and put it in my jeep memoirs!

[addsig]