18

I've come across this term a couple of times in the context of diagnosing engine-related issues, but don't know what it refers to. Here is an example from an automotive forum I frequent:

I gotta say it's not likely to be a burnt valve. These engines seem to be mechanically pretty bulletproof.

So what does a burnt valve refer to and what could cause it?

15

Valves can bend, break and/or burn. If the valves don't provide a complete seal, for whatever reason, the hot gases are forced past the valve which eat away or burn away the edge of the valve due to concentration of heat and pressure.

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Causes that I can think of -

  • Anything that causes incorrect valve seal
  • Cooling issues (improper cylinder head cooling)?
  • Quality of fuel? (carbon deposit on the valves)?
  • Concentration of heat at a particular point on the valve during opening (for exhaust valve) due to improper design.

ADDITION
This is only expanding on the above mentioned points. Mostly cause I can't think of any more

  • Incorrect Valve Seal - for whatever reason (wear, weak spring, valve lash etc) - reduces the quality of valve-seat contact and hence impedes heat dissipation (through conduction to the head). Improper guide and/or loose seat can contribute to this. Also, as the seats wear (choice of materials) and the valves recede into the head, valvelash is lost , which leads to the same, improper contact.

  • Choice of alloy used, width of the stem (for conduction) etc

  • Deposit build up can hinder conduction.

  • Valves can also run hot because of elevated combustion temperatures (for n number of reasons).

  • With reference to the quote in my question, what role does engine design play here? Most of the causes you have described relate to engine operation – Zaid Jun 25 '15 at 9:26
  • @Zaid I will expand a little when I get time. Think about how the valves cool down (~1200 to 1400 degrees F at exhaust) – chilljeet Jun 25 '15 at 11:30
  • @Zaid read my edit. It's far from comprehensive and I'm no expert in this. I suspect the real reasons will be a lot more complicated. – chilljeet Jun 25 '15 at 11:54
  • do you have two spark plugs per cylinder ? – amphibient Jul 10 '15 at 19:45
8

A burnt valve can happen for one of many reasons, but the underlying issue is, the valve is not sealing correctly and allows hot engine gases during the combustion phase to leak past the seal which is formed by the valve and seat. When the seal is lost, the hot exhaust gases escape past the valve (can happen to either the intake or the exhaust valve, but is much more common on the exhaust) and wreak havoc. The burnt valve is usually not the cause of the issue, but more so the outcome of a different issue.

A normal running 4-stroke engine has valves which allow for the incoming air/fuel mixture into the cylinder (or just air on direct injected engines) and for the outgoing exhaust fumes to escape out the tail pipe. Valves in and of themselves could not stand up to the extreme environment of the combustion process. When a valve closes and is sealed against the seat (which is embedded into the head), one of the salvation factors for the valve is heat transference. The head absorbs some of the heat which is collected by the valves. This heat is then transferred out to the coolant system, which takes it away from the area and allows things to run as we consider normal. If a valve, for whatever reason, is not allowed to close completely, a leak in both air/fuel during the compression cycle and exhaust during the combustion cycle occurs. This causes several different problems within the engine:

  • Cylinder power imbalance
  • If leak occurs on the intake side, it causes intake flow issues with other cylinders
  • Burnt valves (due to escaping exhaust gases during the combustion cycle)

A valve can get into non-sealing state for several reasons. These might include, but aren't limited to:

  • Carbon buildup
  • Physical valve damage (broke valve seat, bent valve, etc)
  • Sticking at the valve guide
  • Broke, damaged, or weak valve spring

Any one of these can cause a valve to stay open during the combustion event, which allows very hot gases to escape past the valve. Since it is not allowed to fully seat, there is nowhere for the excess heat to go. It stays at the valve head and damage soon occurs.

  • Many thanks for this detailed answer. This also answers how engine design (more specifically engine head design) had an impact on the likelihood of having burnt valves – Zaid Jun 25 '15 at 21:24
  • will either a cylinder leakdown test or compression test fail because of this ? – amphibient Jul 10 '15 at 19:50
  • @amphibient - It's the primary purpose of a leakdown test. If there is a burnt valve, the leakdown test will show it. – Pᴀᴜʟsᴛᴇʀ2 Jul 10 '15 at 20:07
  • i thought blown head gaskets were the primary purpose – amphibient Jul 10 '15 at 20:09
  • 1
    @amphibient - Same deal. Just remember, no method is foolproof. If the leak is small enough (whether blow-by, burnt valve, or head gasket), it can affect performance, but not truly give any real signs. It will, over time, rear its ugly head so you can see it though. Catching it early is far better than waiting until things get out of hand. – Pᴀᴜʟsᴛᴇʀ2 Jul 10 '15 at 20:26
3

It simply means a half burnt valve thus preventing it from sealing against its seat

When there is no sufficient cooling in the engine it may cause the seal of the valves to give way(exhaust valve generally), gradually over the course of time the exhaust gases escape even without the valve being fully open thus increasing the risk of further damage. A partially burnt valve will be difficult to notice but will produce anomalies in the fuel economy.

Diagnosis:

  • Exhaust valves are generally hotter than intake so they are more susceptible to burning , as stated above , if your valve is burnt then the exhaust gases will escape even though valve is not open, you can do a compression test to adjudicate if the valves good.
  • Cylinder Misfire, if your head gasket is good and still cylinders misfire then there is a high chance that your valves might be burned since it causes compression loss.
  • Slight power loss, depends on the vehicle whether it will be noticeavle or not.
  • Smoke in the exhaust.

Causes:

  • Engine overheating due to insufficient cooling.
  • Engine running lean(one of the common causes).
  • Having LPG kit fitted which on prolonged use causes less lubrication to the valves.
  • i'd hit the answer button before i could complete. You beat me. – chilljeet Jun 25 '15 at 9:11
  • Edit: Bad fuel might also cause valve burning as it leaves a lot of carbon deposit , though i am not 100% sure, I read that shell V-Power somehow helps reduce this or something. – Shobin P Jun 25 '15 at 9:12
  • @chilljeet Ha ha , happpens . its usually difficult to beat the beast paulster , just teleports outta nowhere – Shobin P Jun 25 '15 at 9:13
  • 3
    I think we have to concentrate our efforts on answering questions whilst he's asleep in his timezone – chilljeet Jun 25 '15 at 9:14
  • @chilljeet Thats my technique, you cannnot officially answer any questions after 6pm IST – Shobin P Jun 25 '15 at 9:15

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