I recently got the codes P0303 and P0304 on my 01 XJ with 91000 miles. I had just done a tune up with new plugs and coil pack 3000 miles ago, so I figured maybe injectors? I changed those two injectors and it is stll idling rough and has a misfire. I am being told that it could be that the computer is bad, does that sound right? That is a $1100 part that I don't want to buy if I dont have to. I have checked the compression and it is good so I don't think it is a cracked head. I would appreciate any advice or suggestions.

 

Unfortunately, a compression test is not a good test for a cracked 0331 head.

And DEFINITELY hold off on that computer! A computer diagnosis should be a "diagnosis of exclusion", meaning that everything else needs to be ruled out BEFORE suspecting it. Generally, the computers are pretty hardy. Misfires like you are mentioning aren't a real common symptom of a bad computer. And if you were to get a different computer, never buy from the dealer. $1100 is insane. You get these from a salvage yard for the best deal.

The big question is this. Are you using any coolant? Level going down in the coolant reservoir?

Also, has this engine ever been overheated?

If you answer yes to any of the above questions, you could have a cracked head. Misfires are a common symptom of them.

 

I am not using any coolant, and when I changed the valve cover and gasket a couple weeks ago the head looked good...could it be anything else?

 

Is there a better way to check the head than a visual inspection from the top and a compression test?

 

A cylinder leakdown test is probably the best "test" for it.

Just curious, what plugs are you using? There have been more than a few reports of some 4.0 engines having mysterious misfires with the use of BOSCH spark plugs. I am collecting information about this, but have nothing to share about root cause just yet.

If you do have Bosch plugs installed, try swapping them for NGK. It is the best plug for the 00-01 with the distributorless coil pack ignition.

 

I was using Bosch Platinums, I just heard the same thing from the mechanic who is working on my Jeep. He replaced them with Champion plugs and even tried a different coil just to see and it still has the same problem....

 

just for the heck of it did you replace the spark plug boots?

 

The coil pack came with new boots on it already. I don't want to buy a computer if it is not going to fix it, but I don't know what else to do