Dan Porter

Hope I'm not beating a dead horse here!
My 01 Sport keeps getting a check engine light on that comes back as random misfires.
I have replaced the plugs, the coil pack (twice), the fuel injectors (4 hole) and the cam position sensor.
Compression is around 130 psi on every cylinder.
What am I missing?

5-Speed

You have a year known for having a cracked head issue. Have you been losing coolant without seeing any external leaks? You can go to your local auto parts store and rent a tester that test for exhaust in the coolant system. A cracked head can also cause the random misfire code you are getting.

extrashaky

Cut and pasted from the last time I posted it:

Personally, I would do the free and easy diagnostics first. If you don't have the kits for the tests mentioned below, you can usually borrow them from one of the local auto parts stores that has a tool loan program.

If you haven't already done it, clear the codes, then scan again for pending codes to see what comes back. The computers on the coil rail motors will often pitch a tantrum when there's trouble and initially show misfires on cylinders other than the one that's having the issue. If only one cylinder code comes back, you can focus your attention there. If it remains a multiple cylinder misfire, that can help too.

Check compression. If the compression is low in a cylinder, pour a capful of motor oil into the cylinder and do the compression test again. If the cylinder still has low compression, it's a valve or head issue. If compression rises with the oil, it's a ring or cylinder wall problem.

If you have an air compressor, also do a cylinder leak down test. Many valve and head gasket problems will NOT show with the simple compression test but will show up with a leak down test.

Test fuel pressure at the rail. It should be 49 psi plus or minus 5 psi. Low fuel pressure usually indicates a bad fuel pump. The pump and filter on yours are all one unit, and you would have to drop the tank to replace it.

If you're getting any individual cylinder misfire codes, swap fuel injectors from the misfiring cylinders to good cylinders to see if the misfires move. Inspect the injectors for cracks.

Use a noid light to make sure each injector is getting a signal at the harness. Keep in mind that using the noid light will cause the computer to throw injector codes on healthy cylinders, so don't let them throw you off when you see them. Just clear them and keep going.

Remove and inspect the camshaft position sensor. If the magnet inside is chewed up or you see a lot of debris in there, wiggle the center shaft on the oil pump drive to see if there's excessive play allowing it to wobble and take bites out of the magnet.

Unlike the '99 and earlier XJs with distributors, rotating the cam position sensor doesn't affect timing. The computer controls timing and will adapt to any adjustments you make to the sensor. However, if your timing chain has jumped, the signal from the cam position sensor can be so far out of spec that the computer doesn't know what to do with it. If that happens, you can reindex the camshaft sensor using a toothpick or an alignment cap. Here's a link to that procedure:

http://www.jeepforum.com/forum/f11/h...ensor-1145301/

Your Jeep is also subject to the valve rotation TSB linked below. At higher RPM, the valves in the early coil rail motors may not rotate properly, causing carbon to build up on them. The carbon further prevents rotation and prevents the valves from closing completely. This causes fuel fouling and misfires. Unfortunately it's sometimes such a fine leak that a compression test doesn't detect it (hence the suggestion of a leak down test). The TSB requires you to clean the combustion chamber with Mopar Combustion Chamber Cleaner (but see the next paragraph before doing this) and then manually rotate the valves.

http://www.wjjeeps.com/tsb/tsb_wj_0900303.pdf

HOWEVER, before buying the Mopar Combustion Chamber Cleaner, you may be able to "steam clean" the cylinders enough to avoid having to do the valve rotation TSB at all. This involves pouring small amounts of water into the intake while the engine is running. It's perfectly safe as long as you don't just dump the water in and kill the motor. The steam will remove carbon deposits, and the valves (if they're your problem) will close properly again. Here's a link to the procedure, or you can find plenty of videos of it on YouTube:

https://www.cherokeeforum.com/f30/st...-4-0-a-226299/

These motors are prone to other valve spring problems, so you might also consider pulling the valve cover off and just inspecting for damage. My persistent cylinder 6 misfire turned out to be a broken spring. The spring was causing it to run rich and lean alternately, resulting in a bizarre half-black, half-tan plug.

People always mention the CPS. I hate replacing those, so before diving into that headache, I'd test it with a meter. If you don't have a meter, you can get one for $10 at Harbor Freight that will work fine for automotive testing. Here's a link to the CPS testing procedure:

https://www.cherokeeforum.com/f51/testing-cps-5563/

Up to this point you will have spent very little money, if any. If none of this diagnoses your problem, then you can fire the parts cannon at it and start replacing sensors.

extrashaky

If that's the problem, it usually manifests as a #3 and/or #4 misfire. The cylinder leak down test would also detect it.

extrashaky

Also, if all the free testing fails, you can try replacing your upstream O2 sensors. O2 sensors are lying little bastards that will report to the ECU that they're okay when they're not. The ECU uses data from the O2 sensors to adjust the mixture, so failing O2 sensors can create havoc.

Dan Porter

Not losing any coolant.

Dan Porter

Very informative. Thanks!