tryin' to track down this p0304 on my 2001 Cherokee 4.0...no symtoms at all, truck runs great but as soon as I clear the p0304 it returns withing 5 minutes of run time... the changed plugs, ran Seafoam thru the gas tank to clean injectors...don't have a spare coil pack to try...any other suggestions?...thx

 

Cylinder #4 misfire. Might try swapping #4 injector with another to see if the problem follows. Coil pack problems normally result in multiple cylinder misfires since each coil fires two cylinders. Might check to see that the plug is making a good connection underneath though.
Are you losing any coolant?

 

Do a compression check on cylinder # 4 and a compression loss check on cylinders 3 & 4. That is to say you if aren't losing coolant and your oil doesn't look milky. If you are and it does look that way, say good night Gracy...the head is cracked.

 

ok swapped in a known good coil rail...no help, code returned after 3 minutes of run time...took a compression test came up with 150...next thing to check is a injector swap?

 

Why not. Swap #4 with one of the others and see if the misfire follows or stays.

 

Haven't seen an answer to this question yet.

The reason that this question is so important is that the #1 symptom of a cracked 0331 cylinder head (which is what is on your 2001 from the factory) is "unexplained coolant loss".

Know that good compression numbers doesn't rule out a cracked head. I've seen a cracked head show great compression numbers across the board.

 

Actually having the same code on my 00 xj right now. Ive got new plugs, coil pack, iac, tps, throttle body, 02 sensors, and entire exhaust system has been replaced. I have an older style head too. No 0331 crap. I have the misfire code, but it doesn't feel or sound like its misfiring at all. Shes running great. Im stumped as well

 

Single cylinder or multiple?
As an aside, it seems the 2000+ are way more sensitive to misfires than the older XJ's. The code seems to set if you sneeze sometimes.

 

Single. Just cylinder 4. My thoughts are the pcm doesn't like the big cam in the stroker.

 

no coolant loss, single cylinder...just #4....BUT here is a tidbit that seems unexplained also..when the code first appeared I was getting a 305 and 300 also. So first thing I did was pull plugs and since they were rather new I just re-set gaps on both 4 and 5...well with that action both the p0305 and the p0300 have not reappeared..only the 304. I then swapped out the plug in #4 with no success ...so today I will swap injector and report back ...thx for all replies

 

1. What brand spark plugs are you using? Some plugs like Bosch, Splitfire have been traced to misfires in the 4.0 engine.

2. Whenever I see the P0300 (random misfire), I start thinking along the lines of a "common" component or system that isn't working properly. I always verify fuel pressure for a P0300. For a 2001, you should have 49 psi, plus or minus 5 psi.

 

Im running NGK plugs. Again mines only cyl 4 misfire. Might try cleaning & moving injectors around.

 

todays update ..swapped injectors ...no luck code back on for cyl 4 in minutes. Decided to instead of clearing code with scan tool, to reset computers by disconnecting both battery cables, touching them together, waiting a few minutes and reconnect. Well so far no code...is it possible that one of the remedies I tried actually worked but the computer actually needed to be re-set to remove the p0304? All other times code re-appeared after only 3 minutes...now running over an hour clear...could it be??

 

If you have an OBDII scanner and clear the code you should NOT have to do anything else beyond that to clear the code.

 

And im in the same boat man...cant figure mine out either....but im still running like a scalded dog w/ my 00 xj...i swear the computers we have are sooo picky...