Thread: what is a burnt valve?

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One way to look at it is that it's less than perfectly circular.

It does indeed lose material and can no longer seat properly - properly defined as on the seat the full 360 degrees of seat circumference.

Most times, the exhaust valve burns.
ID'd by mild backfiring/popping when the fresh fuel/air mix gets ignited early.

Leaking intake valves make an engine run rough - as does the burned exhaust valve - but doesn't do the backfiring bit.

Easy to understand why exhaust valves burn when you realize the normal operating temperature for them is red-hot.

Lack of time on the seat - so as to transfer heat - is usually what burns an exhaust valve.

Too many valve grinds on an exh valve make it too thin and create the potential for a burned valve.

If your valve job is fresh and the valves weren't too thin - auto shop machinists will usually reject thin valves so you're probably ok there - you may want to take a look at valve adjustment.
And check compression somewhere along the way.

I tried the towel across the exhaust method you mentioned before (I couldn't find that thread again), I'm not sure if I have burnt valves or not. I don't really have anything to compare the towel action to. The exhaust definitely does not come out smoothly though, it comes out in bursts much more violent than my fuel injected modern cars.

Instead of a towel, I was taught to do the test on the exhaust pipe with a dollar bill....if it always blew away from the end of the pipe, the valves were OK, but if it sucked the dollar against the end of the pipe, that meant that it was going to suck many dollar bills out of your pocket.

ohhhhhh,
ok. I misunderstood the test. I think I'm ok in that case.
I'll try it again with a $20 just to make sure.

welp, I had my truck out this weekend and I figured I'd hold a bill there and see what happens. Long story short, I ended up with half a dollar in my hand, the other half got ripped right off so I guess I have burnt valves.
I'm gonna have to get a valve job on those f'ers, but for the future, is there any way to know what caused this?
I don't want to do all the work to get these heads off and back on again and have this happen again.

This is an old thread but it has the right title for me. My son has a 1992 Lumina with a 60 degree V6 and it has strange behavior which is getting worse. This car was my Dad's from new and we inherited it and then gave it to my son. It has about 85,000 miles on it and it runs good and starts easily. However after running for a while it seems to run out of gas and just sputters to a stop. Our local service folks won't touch it and say we need to go to a dealer for a diagnostic which seems unreasonable to me for a 1992 model. The folks who gave up on it said it might be burnt valves but that does not make sense to me since it starts easily and runs great for a while. I think it is somehow related to heat. The first time we had this problem we were driving at about 70 mph on I-95 in July and it just lost power and sputtered to an inconvenient stop. After it was towed to an inconvenient site it started right up the next day and the ignition was refreshed and things checked out but after we gave the car to my son it has happened more and more frequently. It is a FI model so I wonder if the fuel pump loses pressure when it gets warmed up. Anybody out there have experience with this little Chevy V6 FI engine? I am willing to put money into fixing this rather than buy another used car for my son but it stumps me since two garages have worked on this and come up with nothing so far. Any suggestions?

Don Shillady
Retired Scientist/teen rodder

Thanks DennyW! I would never have thought of the Cat but I know we need to check the fuel pressure. Maybe replacing the FI pump is a good idea anyway with 85,000 miles on the car. Any body else have an idea? DennyW usually has it dialed in and he may be the smartest guy in Illinois! I wish he was closer!

Don Shillady
Retired Scientist/teen rodder

Originally Posted by robot

Instead of a towel, I was taught to do the test on the exhaust pipe with a dollar bill....if it always blew away from the end of the pipe, the valves were OK, but if it sucked the dollar against the end of the pipe, that meant that it was going to suck many dollar bills out of your pocket.

Thanks for that advice. You guys have a lot of info about tail pipe (well in the past ) . But thanks anyway, will keep these info in mind.