04-05-2006 06:56 AM #1
what is a burnt valve?
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?
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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.
Yes, they actually errode away.
Anthing that keeps the valve from seating flat to the head reduces the heat transfer from the thinner material (valve) to the large water cooled head. Each cycle adds more heat, to the point of red hot, and soon begins to melt, causing the valve metal to weld to the head material. The same as when you weld. The way slots, and grooves are cut into the valve, is done by the piston pushing, and pulling the air against the melted areas. This soon makes it like a small torch. It can get so bad, that it begins to burn (cut), the stem of the valve once it starts a hole in the valve itselt. You would know this very soon, because as the valve doesn't seat, and gets worse, the compression becomes less, and causes a miss, or weak cylinder.
If the valve becomes bent by hitting the piston when there is not enough clearance between the valve and piston, or you just over rived the engine to many times, or do what is known as floating the valves, it can cause a valve to burn. Weak springs, loose valve guids, can also cause this.
It boils down to anything that is not in good shape in the valve train can give you problems.
The reason the valve is cut a different angle to the head seat, is to give a reseating action every time the valve seats. (scapping action). On a lot of engines they use the rotators on the exhaust valve to help reseat the valve each time it moves up and down. This helps to keep carbon to a minimim. Stainless steel valves are very popular for this reason also.
So, timing, fuel enrichment, loose valve springs or guids, all play a part in how long you can go before you have problems.
Heres a couple pictures showing burnt valve, hole in valve, and a stem thats got ate away. The second is bent valves.
Last edited by DennyW; 03-12-2007 at 10:54 PM.
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.
Ok, if you have a burnt valve, it will want to suck the towel inward. With no burnt valves, the towel will have a steady pressure outward on it.Originally Posted by tcodi
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.
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.
Be careful, it could suck it in for keeps.
Paper works just fine. A regular piece of paper. It may suck a hole in your $20.00.
With a rag, it will make a popping sound, and you won't be dollar less.
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?
Retired Scientist/teen rodder
Sounds like those shops are not taking the time, and using the test methods to locate the problem.
Fuel pressure test when cold, and as the engine gets up to temp.
Scope test to check spark cold, and when up to temp.
Run vehicle until it quits. Check fuel pressure right then, and check for spark right then.
Check for any computer codes, before the test, and after the test.
One important simple tool also is a vacuum gauge. Check vacuum at idles, and part throttle in park. Once again after the engine is up to operating temp. You want to make sure the Cat is not closing off exhaust flow.
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!
Retired Scientist/teen rodder
Don, I can't remember the exact years, but GM put a plug in resister pack back by the tank area, that gives out when hot. You might want to ask the dealer about the fuel pump resister that is a plug in, and located at the rear of the car. See if it applies to your vehicle. Usually, it will start, then die.
I would still want to test the pressure, and see if the pump was running when the vehicle dies. Remember that the pump will cycle (run) for 2-3 seconds when you first turn the ignition key to the run position.
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!
Retired Scientist/Teen Rodder