What causes a valve to "burn"?

icelandic · Post by **icelandic** » 20 Apr 2009, 14:37

What causes a valve to burn in a normally aspirated engine? And what causes it to break, I mean loosing a smal piece at the valve rim?
Thanks

Post by **billofdurham** » 20 Apr 2009, 16:07

There are several reasons why valves burn. One is normal wear. As miles are put on an engine, the constant pounding and heat erosion wears away the metal on the face of the valve and seat. The exhaust valve sheds most of its heat through the seat, so when the face and seat become worn and the area of contact is reduced, the valve starts to run hot. Eventually the buildup of heat weakens the metal and pieces of it start to break or flake away. As this happens it forms a hot spot that speeds up the process. The valve begins to leak and compression drops. The result is a weak or dead cylinder and a noticeable drop in engine power, smoothness and performance.

A bad exhaust valve will also increase exhaust emissions by a considerable amount because it allows unburned fuel to leak into the exhaust. Thus high hydrocarbon (HC) emissions may also indicate a burned valve.

An exhaust valve can also burn if the valve clearance closes up for some reason (improper adjustment, cam or lifter wear, a bent push rod, worn rocker arm or cam follower, etc.). The lack of clearance in the valvetrain prevents the valve from closing fully, which causes it to leak compression and overheat.

Valve burning can also be caused by any condition that makes the engine run hot or elevates combustion temperatures. This includes cooling problems, abnormal combustion like detonation or preignition, loss of exhaust gas recirculation (EGR), retarded ignition timing or lean fuel mixtures.

Bill.

icelandic · Post by **icelandic** » 20 Apr 2009, 22:14

Bill,

Thanks for the very thorough explanation. I had a broken exhaust valve on cyl 3 in my 94 850 NA at 200k miles. And your explanation comprises exactly what was going on sometime before the valve broke: Poor compression, and problems in the cat converter, perhaps due to the fact that there was leakage around the valve seat.

Thanks again

Icelandic

Post by **rspi** » 30 Jan 2012, 22:22

I found this thread while trolling the web about burnt valves. Some of you know that I'm the "OCTANE" monitor and we all want to prevent problems. Of late, over the past 2 years, I have heard several people saying that a motor was bad because of a burnt valve.

Most of us know that these motors, especially the I5, was built to last 500,000 miles. So why are some of them dieing at 130k and 180k for burnt valves. From what I have found, it has mainly been the NA cars. Over the past week or so I have read numerous thread, articles, ask.com, posting about burnt valves and what people think causes them. Bill really put most of what I read in a nut shell. The thing that stuck out most to me is his last paragraph about timing, pre-ignition and EGR info.

I would like for people that learn of BURNT VALVES to post their conditions here if possible. My bet is that 75% of them are occuring in cars that run low octane fuel. The Volvo I5's and I6's run compression at 10.5:1 and do require 91+ octane. Sure they are made, through the knock sensor technology. to run on 87, however, that messes with the timing and may even cause the car to run a little lean. Running a car lean causes burnt valves. Sure it may take a while but if a car has burnt valves at 170,000 when it was built to last 500,000, you have really cut it's life short.

Yes I also read that overheating, exhaust leaks, and plain old engine wear causes burnt valves as well. However, I would like to know how many are running low octane fuel???

If you had a burn valve in your Volvo, please post your information here. What motor was in the car and what octane fuel did you use? How long did you have the car? If you got the car and 3 months later you learned that you had a burnt valve, you probably didn't cause it. However, if you owned the car for 8 years and was the one fueling it and servicing it, you would have a better grip on the underlying cause.

Post by **jblackburn** » 30 Jan 2012, 23:19

Leaky valve stem seals covering the valves in oil is one of the most common causes on the I5 engines. Those are usually shot somewhere before 200,000.

Post by **erikv11** » 31 Jan 2012, 00:19

+1 to leaky valve stem seals. This will nucleate the hotspots (speckles of burned oil) that lead to the snowball effect Bill describes so clearly.

Maybe Volvo designed the engines to run for 500k, but no rubber part (seal) is going to make that. It starts with the weak link ... Anyway, a burned valve doesn't equal a dead engine, it just equals one that needs a valve job

For the record: bought the NA at 96k, always ran 89 octane, about 170k the valve stem seals started smoking in two of the cylinders (by 180k could see slight oil deposits on those plugs), at 195k had a burned exhaust valve in one of the leaky cylinders.

Post by **tjts1** » 31 Jan 2012, 07:04

A clogged injector will cause one cyl to go lean and over heat an exhaust valves. NA engines use much smaller injectors than turbos and they spend more of their time at stoich. There isn't as much margin for error. ~220k miles (odometer is dead) and still burning 87 octane every day.

Post by **erikv11** » 31 Jan 2012, 07:44

Absolutely, and a Volvo-savvy mechanic told me that clogged injectors are a common source of burned valves he sees in the shop. They key here is, with a clogged injector a valve will burn again in the same cylinder very quickly, say within 5k miles of the head rebuild. And may still never throw a code.

I had the injectors cleaned and flow tested at 150k, consider it a "stage 0" maintenance item.

Post by **rspi** » 31 Jan 2012, 11:06

Leaky valve stems??? Is there a way to avoid that?

Also, I run lucas fuel injector cleaner in my car every 5,000 miles. I know the bottle recommends tossing it in every 3rd tank or something like that (every 1,000 miles), what do you guys think is a good cycle? When I was driving 4,000 miles per month I would run it every 10,000 miles (I only run 91+ octane).

Post by **jblackburn** » 31 Jan 2012, 11:19

Not really. Maybe a good oil will hold it off longer, but it happens to most of the motors around this age.

Mine isn't too bad in comparison to some I've seen pull off from a stoplight in a cloud of smoke, but if my car sits 3-4 days without being driven, it will start with a small blue puff out the exhaust.