Broken exhaust valve
 

A large chunk of the # 6 exhaust valve broke off at highway speed. It was not overheating and the oil pressure was good. It started the check engine light flashing and set off code p0306. I got it home and pulled the plug on cyl 6 and found it to be carbon fouled. Ran compression check and it was zero. I suspected it was a valve or hole in the piston. I pulled the head and found a chunk of the exhaust valve missing. Has anyone had this happen to them and does anyone have a clue as to why it could happen? Engine was running perfect before it broke. I guess I got lucky in that there was no marks on the top of the piston and the cylinder wall was unharmed. I have attached a pic for clarification. Long time user, but first post, so be kind if I made some rookie mistakes.

What are your valves trying to tell you?

Scroll down to "Burnt Valves".

Thanks for the link. I guess what I was looking for is, if #6 is more likely to overheat or run leaner that the rest of the cylinders. It is the farthest away from the egr valve, if that could have anything to do with it. I have converted to the upgraded MPI injectors quite awhile ago, so it is not a sticking poppett valve. Nothing was broken in the valve train and the underside was valve did not have a carbon buildup. I am just trying to see if there is a pattern of # 6 being the most likely for this to happen to.

To the best of my (limited) knowledge, it is not a common problem with the 4.3 engine. I wonder if maybe you had a small vacuum leak at that cylinder or something. The EGR theory seems like a possibility, too.

A vacuum leak on the LIM gasket is a possibility. That should be fixed by the new Fel-Pro LIM gaskets I went back together with. As more eyeballs hit this thread, maybe we can come up with more possible reasons this happened.
I may have to do some spark plug readings after I get it back together and compare # 6 with the other cylinders.

Can you provide an image of the piston surface and the crown?

Valves also burn when debris is stuck between the seat and the valve and hot exhaust gas escapes through the tiny gap. Acts like a blowtorch. Looks as such a failure to me.

Then the reason for such debris can be carbon deposits over time but also be parts of the piston breaking off due to detonation.

Inspect all other cylinders as well.

Sorry, I have already installed the new head. I inspected the pistons on that side and they were in good shape. I dropped the valves on the old head and there was no build up of carbon or oil. I did a compression check on all other cylinders and they were 165-170 psi.

No worries, we engine guys just always make sure we have understood WHY a thing failed.

The piston crown seen from above can yield information about the failure.

Good luck with the new head.

Thanks, If I have a different out come than all good, I will continue the thread.

Ok, I got it all back together and it runs good, but bank 2 is running at -11 on the long fuel trim. If I am correct that is rich. I did have my son drive it 15 miles to an Autozone to get the codes read, so it was dumping raw fuel into bank 2 from cylinder 6 that had zero compression. Also volts on the o2 sensor are jumping from 1.00 to 8.00 back and forth. Bank 1 is + 1 on ltft and .400 to .600 on the o2 sensor. I am thinking I need to replace the bank 2 o2 sensor that may have been poisoned by the raw fuel. What do you guys or girls think?
Also no codes are present.