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2001 4.0L issues


NAXJA Forum User
NAXJA Member
So last year we purchased a 2001 XJ with 4.0L. After owning and driving for about 1000 miles it developed a problem where it suddenly starts losing power and running rough. This doesn't happen immediately, but seems to occur after 5-10 minutes. It runs like a champ and then starts to lose power, sometimes surging as if you were stepping on the gas pedal and letting off quickly several times. The Jeep starts to vibrate as the engine feels like it has lost a cylinder or two. Backing off the gas and trying to keep it moving slowly and eventually it runs normal for a short time before acting up again. We have replaced the coil pack, spark plugs, and fuel pump. The check engine light doesn't work, even running the IP test, but there are also no engine codes when looking at it with a code scanner. Not sure if it is a sensor or what could be causing the issue. Has anybody else ever experienced a problem like this?
Do you have access to a scanner that actually reads values?
As above. I had this happen on a Ford Tempo. Would be doing 60 mph on the highway. Then I'd have to have the petal to the metal to do 50 mph. Then it would go back to normal. Changed the O2 and it was fine. Still, just a guess.
Coil rail failure is rather unlikely based upon the symptoms described. However, there are some basic coil rail testing that can be performed.
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Just an update. The coil pack and spark plugs were the first thing we replaced when the issue started happening. I also swapped out the TPS with another. The other day we took it out for a drive and it was running fine until we had to stop and wait for traffic to clear. It immediately started idling rough and then ran rough as we got it up to 65mph. We drove it a little over a mile and turned off the highway onto a side street. We stopped and turned the vehicle off and then started again. It ran perfectly fine. When we came to another stop sign, my son turned it off and restarted again. It ran perfectly fine the rest of the way home.
My son said it does this running fine thing every time it is shut off and acts up every time you have to stop and the engine idles.

We were able to get the check engine light to work. The previous owner removed the bulb and once replaced, it came on immediately. It did show codes under the ATS and not the engine tab of the scanner and gave misfire codes on cylinders 1 & 6 which we've been told is a common problem. We are thinking it might be an ECM issue since it seems to reset after shutting it off and starting up again.
Going through misfires myself on an '00 XJ. The misfires started on the old motor. Around 280K on it. Then a piston skirt cracked off. Not fun driving 30 miles or more back home. I had been working on a mini- stroker so I finished it up and installed it. It ran OK for a while during initial run in. Then it started flashing codes. Misfire on 6. Can't remember everything that went on. I replaced the coil stick with the Viper coil upgrade. Didn't do it. I did pull the cam sensor shaft (distributor) at some point. The bushing was worn. I had one from a 2001 in good shape. Still, didn't run right. I installed it backward. I found the true TDC, and installed it. I used some plastic fixture that comes with the rebuilt units. I am still dealing with the misfires. I did send the EMC out for repair. Came back with not issues. I used EMC1 out of Tilden IL. Not sure what to do. I think to rotate the can sensor shaft and see if it improves. Also, I installed new injectors. Don't know if any of that helps.

Also, anyone know of a scan sensor that will show the adjustment of the cam sensor? Only thing I come up with is the factory or dealership one. $$$
The Matco will read cam sensor sync but it to cost big dollars!
Any particular model? I see a few on EBay. About $200 to $350.

Also, it was pointed out to me that my electric fan setup might cause the issue. The temp sensor is out of the flow of the coolant. I'll have to check that at some point.
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1 and 6 are on the same coil so I would consider the wiring and pcm driver very suspect.
Problem update. It ended up being a bad O2 sensor. One of the wires was broken inside the insulation. Replaced all 4 with new ones from Rock Auto and now it runs like a champ.
Thanks for "fix".

Too often threads like these are left unresolved.