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.

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.

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.

Yes, they actually errode away.