Need Help w/ Intermittent Engine Problem

I'm a newbie and I've tried searching through the forum but I'm not having any luck finding help for this particular issue. I have a '96 XJ 4.0 4x4 auto with 107K miles and have had intermittent probelems for about 2 months now. It all started with the Jeep just dying while the Mrs. was driving it. I was out of town at the time so I told her to take it to the mechanic. He checked things out and replaced the crank position sensor along with a cam position sensor. The cat was rattling so she had him change that as well since it was in there. He said that he was seeing higher than normal back pressure and that the cat was likely the culprit. That seemed to take care of the issue...for a while.

Now it runs great for a while but if you drive it for more than about an hour, it will cut out and stumble. It has no power to get up to speed and just kind of lunges and jumps when you try to accellerate. The weird thing is that the idle seems to be just fine. It seems like it does this when you are in stop and go traffic and it gets a little hotter but the temp gauge reads just below 210. If I pull off the road and let it sit for about 10-15 minutes it starts fine and the problem seems to go away. I just changed the thermostat as I thought I'd start with the least expensive things first.

I've read a few posts on the throttle position sensor but haven't had a chance to check with an ohm meter. Any help you guys could give me would be greatly appreciated.

~Gaz

I would try replacing the fuel filter and run a can of Saefoam through it.

Any engine codes?

I've had a similar problem with a 93 that had a bad splice in the fuel injection harness, but it was a pretty regular miss, and it threw a code. If no code, I'd incline more toward a fuel issue than an electrical one.

But it might also be useful to run a vacuum gauge on it and see if it tells you anything.

I forgot to mention the fuel and code checks I did. I ran a fuel pressure check when it was running rough and it was at 49 psi (the same pressure I got when it was running fine). I had the faults read earlier and they were all cylinder misfire codes. I changed the plugs and wires and had the codes cleared but the problem is still there.

Any other ideas?

~Gaz

Bad coil?

My 93 was doing a similar thing when it was hot, I had to replace the coil. Not saying that is what's wrong with yours, but that's my experience.

Don't know if this will help....


Check ALL the vacumn lines and ALL the fuel lines from the tank all the way to the engine compartment...make sure they aren't crimped ...you may have a short piece of rubber gasline hose coming from the tank to the fuel line...make sure it doesn't have any cracks..
IIRC your fuel filter MAY be in the tank but I ain't sure bout that

IF you have the quick connect fittings on your fuel line there are rubber o rings (red) on the male part..along with a black plastic spacer make sure the o ring isn't torn or broken...
If I left anything out I'sure somebody will correct me

churky89 said:

Don't know if this will help....

Check ALL the vacumn lines and ALL the fuel lines from the tank all the way to the engine compartment...make sure they aren't crimped ...you may have a short piece of rubber gasline hose coming from the tank to the fuel line...make sure it doesn't have any cracks..
IIRC your fuel filter MAY be in the tank but I ain't sure bout that

IF you have the quick connect fittings on your fuel line there are rubber o rings (red) on the male part..along with a black plastic spacer make sure the o ring isn't torn or broken...
If I left anything out I'sure somebody will correct me

Click to expand...

Not bad ideas, but you did put up a pic of the RENIX coil setup. OBD-I/II XJ's use ChryCo coils, and the ignition control unit is a part of the PCM/SBEC, rather than divorced and mounted on the fenderwell...

5-90

Gaz said:

I forgot to mention the fuel and code checks I did. I ran a fuel pressure check when it was running rough and it was at 49 psi (the same pressure I got when it was running fine). I had the faults read earlier and they were all cylinder misfire codes. I changed the plugs and wires and had the codes cleared but the problem is still there.

Any other ideas?

~Gaz

Click to expand...

Cylinder misfire codes and a reaction to heat?

Coil is one suspect, but wiring is another. I strongly urge you to run a vacuum gauge on this thing and have it set up so it will be visible when the problem occurs. Ignition faults will sometimes show on a vacuum reading, but a fuel injection wiring fault will not show on a vacuum gauge, and this is a very useful bit of information.

If you don't have a useful multimeter, you really should get one. They're plentiful and cheap these days, and one of the things you might end up having to do is test wiring, for which an accurate ohmmeter is needed.

The 93 I worked on that had a similar problem had a single defective splice in the fuel injection wiring harness which showed a small resistance. As it heated up, the resistance increased just enough to shut down one of the injectors.

You don't say whether or not the misfire codes were distributed over cylinders or specific to one or more cylinders. It might be useful to know.

As I understand it, the 96 reads misfires in one of two ways. For electronic or electrical faults in injection system, it will identify the cylinder. For other misfires which don't provide direct feedback to the PCU, it looks for out-of-sync pulses from the crank position sensor, and identifies generic misfiring from these slight fluctuations of engine speed.

5-90 said:

Not bad ideas, but you did put up a pic of the RENIX coil setup. OBD-I/II XJ's use ChryCo coils, and the ignition control unit is a part of the PCM/SBEC, rather than divorced and mounted on the fenderwell...

5-90

Click to expand...

I stand corrected......so is this the right pic...

The misfire codes were across various cylinders and were not occuring on one specific hole. Laws and Miller both mentioned the coil. If it were a coil problem would it manifest itself in an intermittent way? Also, is there a particular coil manufacturer that you guys recommend over any others?

Another question: if the fuel lines were crimped, cracked, or damaged in anyway would I still get a fuel pressure reading at the rail within spec?

It looks like I'll be putting a multimeter on the injector wiring and doing some vacuum testing. Thanks again for the advice and I will certainly let you know if this fixes the issue.

~Gaz

Gaz said:

The misfire codes were across various cylinders and were not occuring on one specific hole. Laws and Miller both mentioned the coil. If it were a coil problem would it manifest itself in an intermittent way? Also, is there a particular coil manufacturer that you guys recommend over any others?
~Gaz

Click to expand...

Yes. One of the common symptoms of a coil problem is an intermittent loss of spark, especially when hot.

Thanks Laws. I'll try installing a new coil and go from there.

Do you guys have a favorite aftermarket manufacturer or is it best to buy from the local Jeep dealer?

~Gaz

churky89 said:

I stand corrected......so is this the right pic...

Click to expand...

Um, not to be difficult, but that looks an awful lot like a late GM setup and module to me.

I don't have a drawing handy, but the only electronics you should find in the distributor would be the camshaft position sensor (CKP or SYNC,) and the ignition control electronics would be in the PCM. The coil is, as I recall, in a bracket on the side of the engine block (passenger side near the dizzy, most likely...)

5-90

Alright...I replaced the coil on Monday and everything was fine until today. I drove home from work (about 45 minutes) on the freeway and went through the normal stop and go traffic getting home and the same stumbling problem reared its ugly head yet again.

Do you guys have any other suggestions? I know someone mentioned testing the vacuum but I'm not sure how you'd set it up to see the test while you are driving when the problem occurs.

Just run a vacuum gage somewhere you can see it, tee it into a "manifold" vacuum line (either T'd into the MAP sensor line, or connected directly to the manifold somewhere) and use enough hose to go from the "port" you're using to where you have the gage hung. Once you do that, you can use any gage (and vacuum hose is pretty cheap.)

I've done this before - as long as your hose is in good shape, you can sneak it out from under the hood in a back corner somewhere.

Another thing - isn't the SBEC mounted somewhere underhood? Heat is the bade of solid-state electronics, and can cause intermittent failures like you describe. I'd also try letting the engine idle with the hood open to see if it happens (idling is a GREAT way to get the engine nice and hot...) and then gradually heat the controller with a hair dryer on LOW to see if that causes trouble. Don't heat past where you can hold your hand on the thing for a few seconds - or take a non-contact thermometer with you, and take its temperature IMMEDIATELY when the engine starts to stall. That's going to be the point you're looking for. (Another advantage of heating the thing up at idle - you can keep an eye on what's going on, you can isolate components that might have heat issues, and it won't kack on you in traffic...)

5-90

Ok...I'm still having the same issues with this XJ. I'll drive it until the engine compartment gets heat soaked and it sputters and stalls out. If I let it sit for about 10 minutes, it starts up again until it gets heat soaked and stalls again. This only occurs in stop and go traffic and not on the freeway.

I've changed the crank & cam position sensors, coil, wires, plugs, thermostat, cat, and checked vacuum (which was good). I also checked the grounds (passenger side near the PDC and the dip stick) and added an additional ground from the ECM to the firewall just in case. I pulled the wire looms off the harness and checked for any wires that showed damage or corrosion and didn't really see any. The wires I could check were right under the hood on the passenger side and included the CPS bundle that runs across the back of the motor.

I'm getting to the point where I'm about ready to trade it in as it is a major PITA. I can't figure out where or what is causing this issue and about the only thing left is to change the ECM (which I'm not sure would fix it either).

Are there any of you that have had similar experiences or know anyone in the Wasatch Front region of Utah that I could talk to regarding this? I'm quickly losing faith in this Jeep.

PLEASE HELP!!!

Thought I'd add this too:

Could it be a TPS or an O2 sensor? Maybe the ECM? I keep getting the same faults: cylinder misfire (various cylinders), CPS sensor errors, etc.

Also, I wiggled the wire harness when it stalls to see if there is a bad ground but no amount of wiggling took care of the stall. I had to wait about 10 minutes before it would start and run.

~Gaz

Here is my shot in the dark for your problem.

O2 sensor

One of my Jeeps is a 96, that would stumble and bumble every so often. It was like the accelerator was disconnected at times. No codes were found, but on a hunch I replaced the O2 and have not had a problem since.

Good Luck!!