'99 XJ 4.0. So yesterday, my MIL came on while going to the store and I could hear a misfire. Pulled codes are P0300, P0303, P0304. It was running fine the day before. I reset the codes and the MIL came back on in two restarts with the same numbers.

Now, since this threw codes for cylinders 3 and 4 at the same time without prior problems, I'm kind've leaning away from causes like faulty injectors (though I'm not ruling them out). I'll be running a compression test on Sunday or Monday (because it's currently 16 degrees outside) to test for a blown head gasket between the cylinders since that's my worst fear.

Any ideas on other things that could cause this? Searching keeps leading me to people with coil pack problems, which I don't have. Plugs, wires, cap, rotor, and distributor were changed out about eight months ago.

 

Compression check will be a good start. And...you'll be able to eyeball the spark plugs while you're at it.

Edit: Head gasket isn't too painful. Ever had a root canal? About the same.... :-)

 

First thing I do for this group of codes is to perform a "fuel pressure test". Must be done with a fuel pressure gauge; no shortcuts. Many parts stores will rent you this tool very inexpensively.

The P0300 is a code that I've seen related to fuel pressure before. For your 99, you should have 49 psi, plus or minus 5 psi.

There are a number of possibilities and yes, a blown head gasket is one of them. But because fuel pressure testing is very easy, I do that first. And after that, I'd perform the compression test. Spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

Good luck and keep us updated!

 

I have the exact problem. I'm going to be following this closely to see what happens.

 

Afraid you'll get your hands dirty or chip a nail?
Why don't you lift the hood on your ride and have at it?

 

Lol because it's colder than a witches t!t out there! I've done the dizzy, wires, plugs, and checked for cracks on the head. I seafoamed to see if it was just lodged crap, no change. I'm waiting to get a open bay in a friends garage to pull it apart.

I meant I am following to see if there is a step or system I didn't think to check.

 

Decided to go with the compression test first because I don't have a fuel pressure tester. Things weren't looking good after pulling the plugs.



Cylinder 2, for a baseline. A little under spec.


Cylinder 3: Damn it.


And cylinder 4: welp.


Well, at least it's nice weather outside.

 

While certainly not good news a little diagnosis beats just simply replacing every sensor you can get your hands on. Is the vehicle loosing any coolant? If so you can borrow a pressure tester from many auto parts places. Might not be a bad idea to pull the valve cover and watch the valve train it may be a wiped cam.

 

If you have the ability to apply compressed air (around 80 psi) to each of those cylinders while they're at TDC compression, you could check to see if the compressed air is leaking into the crankcase (listen at oil filler cap). If so, the rings are probably shot or upper part of the piston is blown out.

Pulling the head to change the gasket may not reveal that the rings are shot. But the cylinder wall may be all scraped up. Need to check.

 

No coolant or oil loss/mixing, thankfully. And true, it could be the camshaft, I'll have to pop the valve cover off to take a look tomorrow since it's raining now.

 

Unfortunately I don't, I live in the city and just did the compression test while parked on the street - no easy way to do a leak down test. The fact that there were no misfires before, and suddenly two neighbor cylinders became problematic strongly points to the gasket, though. Or a cracked head, but that's unlikely because I've never come close to overheating.

Seems like the head's coming off no matter what the case is.

 

What is the casting number on your head? Should be a 06xx or a 0331

 

Hey, I can finally update this! I've been buried under snow forever so I haven't been able to do the job until now.



Blown between 3 and 4.