Repeat valve burn

[Jeepster2586]

I hoping someone here has some good ideas on what might be cause my number 6 exhaust valve to keep burning. I have a 97 TJ with a 4.0. Last fall the Jeep starting misfiring terribly, 306 code. I diagnosed a burned exhaust valve on cylinder number 6. I replaced the head with a remanufactured one. After the replacement, the Jeep ran great and all was well again. Then after about 500 miles, and at the top of the MA Bell trail, check engine light came on, 306 code again and it was misfiring. The number 6 cylinder exhaust valve burned again. Does anyone have a clue why the number 6 cylinder keeps burning valves?

The engine has not and does not overheat runs right around 210 all the time, the injector is new on the #6 cylinder, new distributer, new cam sensor, new O2 sensors, new catalytic converter, new crank sensor, new PCU, new wires, new coil. Compression 150lbs on all cylinders except for #6, which after the head replacement was back up to 150lbs like the others.

[Richl35]

I hoping someone here has some good ideas on what might be cause my number 6 exhaust valve to keep burning. I have a 97 TJ with a 4.0. Last fall the Jeep starting misfiring terribly, 306 code. I diagnosed a burned exhaust valve on cylinder number 6. I replaced the head with a remanufactured one. After the replacement, the Jeep ran great and all was well again. Then after about 500 miles, and at the top of the MA Bell trail, check engine light came on, 306 code again and it was misfiring. The number 6 cylinder exhaust valve burned again. Does anyone have a clue why the number 6 cylinder keeps burning valves?

The engine has not and does not overheat runs right around 210 all the time, the injector is new on the #6 cylinder, new distributer, new cam sensor, new O2 sensors, new catalytic converter, new crank sensor, new PCU, new wires, new coil. Compression 150lbs on all cylinders except for #6, which after the head replacement was back up to 150lbs like the others.

Typically what burns an exhaust valve is a slight sealing issue on the valve to valve seat surface that allows hot centralized exhaust gasses to continuously pass which overheats and eventually erode/melt the valve. But saying that, anything that may hold the valve open will cause the same thing. So a sticking lifter, rocker arm assy, weak valve spring, slightly bent valve etc.. can all lead to a burnt valve.

Other things that can cause burnt valves are cooling issues with that section of the head or too the head? Clogged cooling jacket or limited flow can allow the valve to over heat and burn up. Lean mixtures, detonation from improper timing, cheap fuel in a high compression engine (unlikely I know) but unfortunately there are a few reasons.

[Jeepster2586]

When I replaced the head after the first valve burn, I replaced all of the lifters too. The only thing that makes sense is a cooling problem around the effected cylinder. I'm going to pull the head again to replace the valve, and inspect all of the coolant jackets. The problem is only one cylinder and always the same one so I don't think it is cheap fuel, improper timing or anything that would effect all the cylinders. When I replaced the head the first time, it ran like a champ afterwards, so I THINK that rules out practically everything except for a cooling problem on the cylinder, right???? I'm loss for answers.

[Richl35]

It's tough to say but a starting point. It would make sense that whatever the problem is, cooling or whatever, it is isolated to that cylinder. I would dykem the valve seat and new valve face when you assemble the head and lap them in. At least you have a visual verification you have proper seat contact on both the valve face and seat.

[BlackNBlue-ISH]

Could it be something to do with the camshaft? Bad cam eventually causes failure of some sort on the pushrod, which then causes valve to stick open?

[ZAEDOCK]

Did you replace the rocker arms?
The valves spin in their seats during operation and a screwed up arm can stop the rotation.

Typically, people remove the valve train and reinstall in the same location, hence the problem hasn't moved with the new head.
I think I have a few rocker arms kicking around I could mail you if you need it. Let me know.

These mystery problems suck, I feel for you. Good luck man!