Hey Everyone...first time posting. Thank you in advance for any help you could provide chasing down this misfire.

I have a 2000 Cherokee, everything is stock. I have a P0301 cylinder #1 misfire code consistently coming up that is hard to peg down...here is what I have done:

New plugs
New Coil Pack rail
New injectors
Tested for spark out of the rail
Tested for spark on the plug
Tested the Fuel Injector pigtail with a noid light
Replaced all 4 O2 sensors
Checked for Vacuum leak (none found)
Checked Compression and cylinder 1 was at 125 (dry) compared to other cylinders which were between 140-150. (even though #1 is lower...it's still within safe range and leak down test didn't find anything).
Did Leak down test and cylinder maintained pressure...no loss. In any cylinder
Replaced Crankshaft Position Sensor

Any ideas? I suppose it could be ECM, though everytime I fix something and clear codes that Cylinder one consistently comes up. It is def. misfiring...I can feel it and hear it...though just don't know why.

Thanks again for anyone who may have had something similar or can point me in the right direction.

Regards,

 

I would move the #1 spark plug to #2 and the #1 fuel injector to #3 and then drive it and see if the miss fire moves . if it does then you know what it is , if it doesn't then you know what it isn't , then you can look at valve springs .

 

Hmm. If it were random, and an older XJ with a distributor, I'd say look at the cam position sensor. But, it's not random, and you don't have a distributor. There's got to be an equivalent sensor, though.

But it's not random, and you have replaced and tested all the primary suspects. The only thing you have found wrong with #1 is the compression.

Can't hurt to give it the MMM treatment.


This guy used some compressed air to force it into the rings, but most folks just soak it overnight, or maybe longer. Can't hurt!

 

- Thanks tried that...same result.

- Thank you...I will look into that.

Any other ideas out there??

 

The next thing I would look at would be the valve springs , you could have a weak spring or a bad rock arm .