ShrimpBurrito

I have read dozens of the misfiring threads on Jeeps, but like many prior posters, my situation appears to be a bit unique from the others. I'm hopeful some wise folk here can point me to some things to narrow down the issue.

My 97 XJ Sport (6 cyl, ~171,000 miles) starts like a champ -- it was hesitating recently, having to turn over for several seconds before starting, but my replacing the plugs (Champion copper), wires, distributor cap and rotor fixed that (let's call that the "tune-up"). Parts all NAPA brand, their high-end line (Echlin, I believe). It is what I used for the last tune-up.

The tune-up completely resolved the starting hesitation, seemed to give the car more pickup, results consistent with prior tune-ups, but surprisingly, it also immediately revealed a failing muffler (especially during quick accelerations or decelerations), which had previously been completely silent.

However, shortly after doing that (that same afternoon), while the starting hesitation was completely resolved, it started sputtering/misfiring fairly consistently, something it had previously done only occasionally. Instead of only once every few days, it was doing it a few times per short drive. It has since gotten much worse over the last several weeks, but the sputtering/spitting/misfiring still only happens while under acceleration and under load, like going up a hill. In fact, I can often make it stop in its tracks while going up a hill if I just let up on the gas. It happens far less while going on level streets, has happened only a few times while idling, and has never happened during my repeated attempts to make it happen while parked and revving the engine up to 3k RPMs. It happens when the engine is cold or hot, although it "may" get better after getting hot. I haven't driven it enough yet to notice a consistent pattern.

I can't tell if the tune-up accelerated a progressing problem, created the problem, or was completely coincidental with an inevitable problem. I don't think I screwed up the tune-up (I've done it several times before, both on this Jeep and a 93 XJ), and I did measure the gap on every plug (0.035").

Right after the tune-up, but after the seemingly worsening of the symptoms, I cleaned the throttle body and IAC for the first time (they were filthy) and replaced the air filter. I have since tested the original ignition coil, which was fine, and checked the fuel pressure, which was unwavering at 45 psi even during the hiccups I encountered while driving. No overheating, and no oil in the coolant overflow tank. It happens when the engine is cold or hot. Also put some 3M fuel injector cleaner in, but after running the entire tank, the problem remains.

Eventually, the CEL came on, and I read the following code:

P1391 - CKP or CMP Sensor Signal Intermittent

So that might point to a faulty crankshaft position sensor (CPS), but it doesn't seem consistent with what I'm experiencing. If it were a faulty CPS, I wouldn't expect that just letting up on the gas would immediately and predictably get rid of the sputtering/loss of power. And further, when I had a CPS go out on my '93 XJ, it just wouldn't even start at all, an experience I have since read is similar to what others experience (in addition to stalling).

I did replace the camshaft position sensor (CMP) (NAPA/Echlin line), thinking that maybe I upset something when I replaced the distributor rotor/cap, and that did seem to improve things quite a bit, but it has since re-worsened, although now it's barely driveable (whereas before I wasn't even sure I would make it home).

Downstream and upstream O2 sensors test normal with a multimeter, as does the engine coolant temp sensor. MPG has plummeted, now maybe 10 MPG, or roughly half what it has been for the last ~20 years. That may be why the ~2-week old spark plugs have carbon all over them, as pictured below. I pulled them to double-check the gap, and then when that didn't reveal any issues, I replaced them with an identical set, thinking perhaps that QA/QC in India perhaps wasn't as good as the US factory (the Champion plugs are now made in India).

I also disconnected the battery overnight with the idea that might clear the computer and force a re-learn of the air/fuel ratio settings.

Any suggestions? I would be thrilled to get some feedback, and I promise to report my solution!

Many thanks,
Dave

ShrimpBurrito

BTW, after I cleared the CEL, it didn't come back on for another week or two. Cleared it again, and it has not returned.

Dave

ShrimpBurrito

And no obvious wiring issues around the connectors, at least in the crankshaft and camshaft position sensor circuits, but I haven't pulled off the wire looms to inspect the covered wiring.

Dave

ShrimpBurrito

Right after replacing the plugs, I flashed the onboard computer using the positive-lead to AC condensor, ignition on, headlights on then off procedure. That immediately fixed all symptoms, and I could not make it misfire/sputter for anything, even using the previously surefire ways like going up a steep hill with a cold engine. It remained completely resolved for 2-3 days, after which it started misfiring/sputtering again, but with FAR less frequency. Maybe 3-4 times within 10 min after a cold start, and then maybe 1-2 times more during a 30-min drive. It has remained that way for the past week or so.

Not sure what is going on exactly as the computer "relearns" the fuel/air ratios, etc., and why it functioned perfectly under the factory settings, but then has resumed under, presumably, some level of learned settings.

Any ideas?

Thanks!
Dave

CCKen

You may want to test your coil (or replace it). Stumbling under load is a typical coil fault.


You may have to remove the coil to get a the pins properly. While it's out, inspect it for cracks. Any crack is cause for rejection.



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ShrimpBurrito

Thanks for the suggestion! I had replaced a coil in a previous XJ, so I had actually already tested it, and it was fine. But I also put in a NAPA brand replacement, and that didn't resolve the issue, so I put the OEM one back in.

Dave

ShrimpBurrito

Anyone? I'm stumped. Other than to start replacing all the tune-up parts with MOPAR parts, I don't know what else to do. It just seems odd to me that some sort of a reset to the ignition, either via a computer memory flash or perhaps through replacing the camshaft position sensor, both fixed the problem temporarily. And the memory flash fixed it completely for several days.

Thanks!
Dave

Garrett S.

Might be a computer problem but I wouldn't go that route until everything else was eliminated for sure. Had a 90 something dodge stratus in at the shop and it did something similar. It would randomly start running terrible almost die. we tried plugs wires coils 02 sensors new cat we ended up replacing the computer and that fixed it. But like I said that's a pretty major part and I wouldn't mess with it unless it was the last possible option.

Maya Akers

I'm having issues with my 88 XJ Chugging and sputtering in what feels like first gear. If I go over 5mph it happens then once in second it clears up. I had my CPS replaced 3 months ago so I'm assuming it's not that... buuuut who knows. Hopefully I can lurk through the forums and figure it out.

Click-Click-Pull

Check the camshaft position sensor wires in the plastic wire loom and inspect the back of the connector plug. I had a sim problem a few years ago where the wire insulation on two of the wires failed and created a short there.

jordan96xj

This is one of those times when a $25 BAFX OBD2 scanner can come in very handy. It can provide insight into what the computer is doing. Particularly, fuel trims, open/close loop status, and O2 voltage swings.

As long as its safe to do so, next time it stutters under load, try depressing the gas pedal fully (for a few seconds at least). Report back whether that immediately causes a change in behavior (e.g. does it stop stuttering and come to life).

Johnson98xj

My 98 is doing the same thing, only under a load. I've changed the plugs and plug wires so far and I have a new fuel pump that I'm trying to get put in this weekend.

PatHenry

Also check your coil as suggested in the previous posts.

season818

What was the solution to this issue? I've been having the same issue for years, and my plug, wire, distributor cap and rotor tuneup just made it worse. The hesitation is usually resolved by stomping on the gas pedal. I've already replaced the above, TPS, exhaust manifold, and O2 sensors.

mannydantyla

It could be a fuel pressure problem. Check the fuel pressure and then park the jeep on a steep hill and rev it up and check the fuel pressure again. Just an idea

The thing about going up a hill makes me also wonder if its a vacuum related problem. When I go up a hill, my heater/ac fan stops working right because the vacuum canister looses its vacuum, probably because the manifold vacuum goes down when the skinny peddle goes down and the checkvalve between the canister and the manifold is broken.