Engine misfire/stumble sometimes at idle, sometimes under load...

I have a strange problem and need guidance on where to start. My 94 XJ, 4.0L/Auto all stock engine, was running like a top while DD-ing it for about 6 months (usually only weekend play dates prior to that but ran fine) until the day I went to PU another Jeep and then it started acting like it had a plugged cat. I went ahead and parked it figuring I would get around to changing it eventually and now 6 months later I did.......
Symptoms when I parked it were that it had a high speed miss/stumble kind of feel to it but would also feel like it was missing under aceleration from an idle. This all came on suddenly with no problems leading up to it.

Anyhow, i let it sit the last several months since I didn't need to drive it. Recently I finally took care of other issues like replacing a badly dented gas tank (replaced last weekend) added a tank skid, tune-up w/ plugs, fuel filter, oil change in preperation for a smog check I am overdue for.

Ran it around town this morning for several miles of stop and go traffic, short little high speed blast down a stretch of road, stopped for gas, put a few gallons in, ran it to the muffler shop and it ran great during that whole trip, about 10 miles and 20 minutes worth.

Picked it up from the muffler shop where they installed a new cat and tail pipe. Ran fine for the first few miles, about 15 mins worth in stop & go traffic, sitting at lights, etc and then all of a sudden it started acting weird again, stumbling under acceleration and slightly at idle in gear but seemed smoother in neutral...... UGH!

What would you XJ pros check? I will be looking at easy stuff like the cap and rotor tomorrow but figure that wouldn't cause intermittent problems.
Help!?

Crank position sensor?

I'd look for a bad wiring connection, loose, or a bare hot wire or signal wire, that is grounding out intermittently. Wiggle the wires while it is idling as a test.

Also might be the O2 sensor or its wiring.

Did you replace the wires, cap and rotor?

X2 on CPS.

All the sensors under the hood can be tested with a simple volt / ohm meter!!!

Ecomike, I will be trying all those things today and picking up new wires, cap & rotor (checking them they looked fine and only have about 10K miles on them though) Would the CPS cause an intermittent problem like I am experiencing?

Is there a good source (other than FSM) for sensor testing info on my 94 like I keep finding for the Renix engines? I'm off to keep searching but spending a lot of time only finding Renix stuff.

Thanks for the ideas so far everyone! BTW, i didn't mention it in my 1st post, engine has 185K on it. Also, it sounds like I might have an exhaust leak but it sounds like it goes away once it warms up. Perhaps it opens back up once hot and is fooling my 02?

When my O2 sensor crapped out, it would misfire and stumble, even a backfire 'pop'. Usually happened after a hot restart, then it would run fine. New sensor fixed it.

jjmat3 said:

Ecomike, I will be trying all those things today and picking up new wires, cap & rotor (checking them they looked fine and only have about 10K miles on them though) Would the CPS cause an intermittent problem like I am experiencing?

Is there a good source (other than FSM) for sensor testing info on my 94 like I keep finding for the Renix engines? I'm off to keep searching but spending a lot of time only finding Renix stuff.

Thanks for the ideas so far everyone! BTW, i didn't mention it in my 1st post, engine has 185K on it. Also, it sounds like I might have an exhaust leak but it sounds like it goes away once it warms up. Perhaps it opens back up once hot and is fooling my 02?

Click to expand...

Yes, weak CPS or weak CPS signal from bad wire or connections is a possibilty, but so is O2 sensor and ignition components...etc.

Send me your email address by PM.

well I am gonna go ahead and embarrass myself now by admitting that I overlooked the simple stuff....... You know how it is when you least suspect the most obvious stuff so you look for more difficult parts to diagnose, replace, etc?? Come on, admit it, you KNow what I'm talking about right??

Well I started poking around the distributor to remove the cap and check it and the rotor (only a couple years old and Maybe 15K miles on them total) when I pulled the cap off I saw that the coil wire boot was still attached to it, upon looking closer, i see that the coil wire looks like someone had taken a torch to it and burned it from teh inside out.....WTF? Never in 25+ years of working on cars have I seen a coil wire look like this:

There was powdered, ash like coil wire falling out of the boot too. I am amazed it ran at all like this especially considering how well it did run just prior to acting up.
Anyhow, I also noticed some funkiness around the coil terminal and some cracks in the casing so I shotgunned the bitch w/ new cap/rotor, wires, coil, O2 sensor and looked over everything else I could get my hands on to see if anything looked obviously wrong or bad.
Took her for another spin and she ran Great again. Hopefully not just giving me a false sense of security again like last time. I hope she is no finally ready for the smog check! Anyhow, I learned once again to always check the obvious first even if you feel it is a waste of time.....since not doing so Creates a waste of time. (I'm still looking for those sensor testing procedures, so I'll be hitting Eco up back channel)

Thanks again for the help and hope someone else learns from my mistake.

make sure when you go and store your vehicle for several months that you add a stabilizer to the fuel, fuel over a few weeks actually turns to a shellac or varnish, which in turn will clog up injectors, needle valves etc. Also even though gas prices are screaming, use the middle grade of fuel, you'll see an improvement in gas mileage and also an increase in power. Plus the higher grade fuels have detergents that will clean all the gunk etc. out of the injectors, needle valves, depending on your fuel delivering system. Also use some STP GasTreatment, that will remove any moisture within your tank as well as clean out the fuel system. Simple things make for less aggravation later on, be well

Looks like it was arching a spark off the block or a piece of metal somewhere to me, bad coil wire, go to your room, lol