I searched high and low on the web while trying to sort this issue so I figure I'll share and hopefully save someone else some time.
A few months ago my 1996 xj 4.0 aw4 started having a rough idle and p0300, p0301 codes. Other than that seemed to run fine. I tested/cleaned injectors, tested fuel pressure, tested compression, replaced the plugs, wires, cap, rotor, coil, distributor pickup, CPS and nothing was solving it or out of spec. Talked to a friend that owns an auto repair shop and he said burnt exhaust valves are common on 4.0s coming through the shop. Do a running compression test and I'll probably see an about a 15psi difference on the cylinder with the misfire code. I'd not heard of a running compression test before but it's the same as static test except you start the engine and let it run. When the engine is running use the release on the pressure gauge and let it build pressure again while you watch it because the initial cranking to start can give you a false reading. I did this and my number one cylinder was 65psi and the others were 80psi. I then took my head to a machine shop and sure enough number one exhaust valve was toast and the other exhaust valves were far enough gone they needed to be replaced as well. Ended up having all new exhaust valves and hardened seats installed and the engine runs like normal again and no codes. The machine shop also said they see a good amount of 4.0 cylinder heads with bad exhaust valves. Not sure if it makes any difference but mine is the 0630 head.
A few months ago my 1996 xj 4.0 aw4 started having a rough idle and p0300, p0301 codes. Other than that seemed to run fine. I tested/cleaned injectors, tested fuel pressure, tested compression, replaced the plugs, wires, cap, rotor, coil, distributor pickup, CPS and nothing was solving it or out of spec. Talked to a friend that owns an auto repair shop and he said burnt exhaust valves are common on 4.0s coming through the shop. Do a running compression test and I'll probably see an about a 15psi difference on the cylinder with the misfire code. I'd not heard of a running compression test before but it's the same as static test except you start the engine and let it run. When the engine is running use the release on the pressure gauge and let it build pressure again while you watch it because the initial cranking to start can give you a false reading. I did this and my number one cylinder was 65psi and the others were 80psi. I then took my head to a machine shop and sure enough number one exhaust valve was toast and the other exhaust valves were far enough gone they needed to be replaced as well. Ended up having all new exhaust valves and hardened seats installed and the engine runs like normal again and no codes. The machine shop also said they see a good amount of 4.0 cylinder heads with bad exhaust valves. Not sure if it makes any difference but mine is the 0630 head.
