2001 jeep intermittent misfire.

[RetEBoogie]

I recently bought a 2001 jeep Cherokee with 140000 miles on it. It's decently clean and I can't wait to drive it. Only problem is I have a misfire that appears to be random in nature. I can walk out and hit the key misfire, shut it off and try again misfire. Other times I can start it and it runs like a champ. I'm getting frustrated and would like some guidance. I have replaced the coil rail wiring think it may have been broken, I've done a new coil rail also. The electrical connectors have been cleaned in the engine bay as well as grounds. It seems to be cylinders 1-6 so I'm at a loss. The previous owner replaced the cam sensor, fuel injectors, fuel rail and the pump allegedly. Thanks in advance for any help.

[Unclewolverine]

I can't remember if 1and 6 are a pair for firing, but if they are its probably still the coil rail.

[TheXJDrew]

Check your injector wiring harness. I had a similar issue recently with a TJ. The wires for the injectors under that big plastic piece that snaps down onto the head bolts had degraded over time and were grounding out on themselves intermittently. Some fresh injector wires and connectors and he was good as new.

[RetEBoogie]

I appreciate the info. I have done the fuel injectors clips and inspected the harness, all looks good. I just don't understand how it runs great for a cycle and the misfires bad, popping from the exhaust and ever now and then from the throttle body.

[IslanderOffRoad]

Cold starts or hot starts?

[RetEBoogie]

Any starts it's completely random. It's running right at this moment but I can start it and there is a misfire. I am aware of the vapor issue and I'm almost sure that's not the problem. I have the hood of at the moment.

[RetEBoogie]

Also the tach is kinda jumpy when it misfire.

[RetEBoogie]

Originally Posted by RetEBoogie

Also the tach is kinda jumpy when it misfires.

[RetEBoogie]

So I do have an update. Walked outside started the Jeep and it ran fine drove it around the field and it ran fine the entire time. Come back to the house turn the Jeep off let it cool completely and now when I fire it up I can rev it to about 2,500 to 2,000 RPMs and it backfires through the exhaust. This is what it has been doing at any point in time whether it's hot or cold. All injectors were tested and all were within normal limits.

[Tim_MN]

How does the Crankshaft Position Sensor and the Camshaft Pickup Sensor synchronization look like on an oscilloscope ?

A stretched timing chain or incorrectly synced Cam Sensor will have similar symptoms to what you describe.


Have you read this >> https://web.archive.org/web/20161020.../pdfs/jcss.pdf

[RetEBoogie]

I'm unsure as I do not have one to check.

[TheXJDrew]

Another issue I have seen more than a few times is the upstream o2 sensor (the primary one right below the exhaust manifold) will go bad and usually it doesn’t throw a CEL, but it will pop the fuse under the hood for the oxygen sensor. It will cause a really weird misfire and it will be totally random. Put a new fuse in and it’ll work for a short time, then do the same thing. Usually causes the backfiring like you mentioned.

[RetEBoogie]

Thanks TheXJDrew. I just went outside and checked the fuse for the O2 sensor and they look fine. I did the camshaft sync with the tooth pick method and still does the same thing. I was going to check fuel pressure today but the rail installed prior to my ownership does not have a spot for the gauge. It was suppose to have a new fuel pump also but I'm not 100% sure. I'm kind at a loss. Also has anyone ever experienced a faulty pcm do this? I may be able to get another ecu but it will be from a 2000 and I'm not to sure it would plug and play.

[RetEBoogie]

So today the jeep started and ran fine. I turned it off about 3-5 mins after and now it is misfiring again. I checked all connections and all was good. I'm not sure where to go with this thing anymore.

[RCP Phx]

When my 2000 started having similar issues, it was the wiring cross-shorting between cylinders and eventually took out one of the drivers in the pcm.