Ok here we go I did search on here couldn't find any answers I was looking for. My 00 XJ 4.0 is throwing DTC P0303 #3 cylinder misfire I have read about the "heat soak" issues and I don't think this is case I have done the usual fixes for the heat soak thermal wrap on the injectors, spaced the hood and put a thermal relay on the electric so it runs when the jeep is off. The misfire is constant too not just at idle. To try to fix this I have changed the plugs regular AC delcos, swapped injectors, swapped new plugs, ohmmed out the coil rail checks good, tested the injector harness w/ noid light pulse looked good same as other injectors, Checked for a trigger signal from the camshaft position sensor pulse looked good, Did a cylinder leak down test #3 has a small intake valve leak was within the range of all other cylinders. Am getting ready to do a compression test today couldn't find my compression gauge last night while I was working on it. I am really aggravated at this thing is there something I am missing something small I have overlooked any help would be greatly appreciated.

 

did you check the hightension cables ?

 

You have addressed the usual suspects. See what the compression test shows. Run it dry, then wet.

 

Ok so I ran the compression test today results.....well aren't what I wanted to see very lower pressure on the #3 80psi and the #1 and 2 were low too. So the fun begins.

 

Probably gonna get bashed for this what are the hightension cables?

 

You should run a wet compression test too. That can help you determine if it is the head or the rings.

Add a teaspoon of oil, re-run test. If compression comes up, the rings are suspect, as the oil will help temporarily "seal" the rings.

 

I know it is a pain but running more than one set of compression readings will give you the truest picture of your cyl conditions.

 

the eletric wires from the cap to the sparkplug. maybee i spelled it wrong, im from denmark

but with the misding compresdion on cyl3 that might be it.

 

Yeah I did run the compression test 3 times took the average of all 3 test as far as the wet test only ran it once. I know its the rings and head is going in for machine work it has some small leaks on the intake and exhaust side nothing major but since its coming off its going in.

 

The 00 xj dont have distributors or plug wires. Its just a long coil pack and a cam position sensor.

 

ohh my bad i wasent paying attension to the year.

 

OK I know my thread is a couple months old but I just wanted to let you guys know I rebuilt the top end on my 4.0 had to replace the head also it was cracked. It ran good for a while then found out that both cats were clogged on the y pipe(cali emissions). Replaced them now I have a new wonderful problem a P0306 #6 cyl misfire I have no spark coming from the coil anybody have any suggestions for this?

 

Have you swapped coils to see if the missfire follows the coil? If it follows the coil you know you have a bad coil

 

I found the problem it was the coil rail it was really weird the coil rail ohmed out correctly but was bad. Swapped it out with my buddy's coil rail and works just fine.

 

I had my 2000 diagnosed and its a cylinder #3 injector misfire. Thee mechanics I took it to, they arnt being very helpful to fix the problem. What would you guys do first to try an fix the problem?