Hello everyone,
My 2001 jeep xj has been giving me some concerning engine codes (P0300, P0301, P0303 and a upstream O2 sensor but I can't remember which one) and coolant loss. I've only replaced the fuel injectors and spark plugs so far, I'm trying to figure out if the issue is bigger problem like a cracked head because of the loss of coolant. I've read a lot about the 0331 heads cracking between the 3-4 cylinder and want to make sure that when I'm replacing parts, my money is being used on the correct part to solve the issue. To diagnose the issues, I'm planning on testing the coil for spark, checking compression, checking fuel pressure, pressurizing the cooling system and using a camera to see if I can see any cracks/coolant between cylinders 3-4. Is there anything else I may be missing? This is my daily driver and I need to get it going so I can get to work on the comanche I bought on Saturday!

 

What did they plugs look like?

Coolant pressure test is good. Also check for exhaust fumes in the coolant. If you can, do a leak down test.

 

Thanks for replying Saudade, the spark plugs looked ok, they needed to be replaced but nothing very alarming. I did it about a year ago tho so I'll check their condition when I do a compression test. The leak down test is also one I'll do. Seems like if the head is cracked, it might be going through the exhaust because I don't see any signs of it in the oil.