Thread: what is a burnt valve?

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When a valve is burnt what actually happens to it that screws it up?
Does it get warped or something? Or does some of it actually erode away?

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.

Yes, they actually errode away.

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

DennyW, Thanks for that comment, it is worth checking out. I have been reading other stories on the Internet about 1992 Luminas and there are a lot of similar stories, some of which are due to the trans lock up stalling the engine by not downshifting so maybe a trans flush is in order but that doesn't explain the cutoff while running at 70 mph so I think your suggestion is worth checking out. The pump is inside the tank and is electric so it is a hassle to change but the external resistor should be easy to check. Your repair experience is priceless! The truth is that most of my experience has been in building modified engines, not very well, and then then not worrying about the fact that most of my rebuilds seldom made it to 50,000 miles now that I think back so the family car as a transportation device with high mileage is a bit beyond my experience. Even so am I right that the symptoms do not match a diagnosis of burned valves? If the valves were burned it would not run well at all? Note that these later models get pretty complicated and it will only get worse if/when hybrids become common. Thanks a bunch, you are a gem!

Don Shillady
Retired Scientist/Teen Rodder

Originally Posted by Don Shillady

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

Don, I agree with Denny. Put the vacuum gauge on it and go for a ride down the highway. If vacuum slowly moves lower and lower I think you'll find your problem in the exhaust system. The 2 things I've seen most are a failed (plugged) converter and more rare but it happens, a collapsed pipe. I've seen a few pipes that have a inner pipe and it expands inside the outer pipe, without any room to expand, it collapses into the pipe interior, effectively blocking off any flow. No flow out = No fuel in.... I think you'll find your problem by measuring the vacuum. My 2 Cents! LOL

Originally Posted by Don Shillady

DennyW usually has it dialed in and he may be the smartest guy in Illinois!

Don Shillady
Retired Scientist/teen rodder

OMG Don you've done it now, this will go straight to Denny's head & he'll believe that ..joe

Just a report on this situation after allowing some time to see if the "fix" works. It turns out that we decided the first thing to check would be to flush the transmission since that was not done for over 25,000 miles. To my amazement the folks who drained the trans said it was 3 quarts overfilled! They said the symptoms we had could be caused by too much fluid. I wondered why it would not overflow but apparently there is room for that much trans fluid to be sealed in without burping out. Anyway after the flush and adding in the correct amount of fluid the symptoms have not returned, although my son still does not completely trust the car. Even so I think this simple mistake was the main problem. Why didn't the extra level show up on the trans dip stick? I thought I did check it "hot" but ultimately I did add the extra on more than one occasion thinking the trans was leaking and probably the trans was not hot enough to give a correct reading on the dip stick, but I find it amazing that I could not see the extra three quarts on the dip stick!

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.