What causes a burnt or broken valve?

[smac]

I bought a sick Daihatsu Copen to fix up for the Misses. No compression on Cylinder no4. Taken the head off to find quarter of one of the exhaust valves missing. No sign so far of the missing bit - the search continues. No other apparent damage. I dont know whether it was a mechanical failure or if the valve has burnt away - im not sure how each would look? Any thoughts on what could have hapenened? Its the 660cc turbo.

[Turn7]

I stand to be corected, but I always thought lean running caused valves to run hot and burn. That and too much advance.

[BliarOut]

Insufficient clearance can also do it.

[stevieturbo]

As above, too lean, timing retarded, incorrect valve clearances etc etc.

Basically anything that can cause excessive heating of the valve.

[smac]

Is the butnt more likely than the broken? Is there a way to tell which?

[stevieturbo]

smac said:

Is the butnt more likely than the broken? Is there a way to tell which?

Pieces of valves dont break, and yes, just look at it.

[MG CHRIS]

Common on gas converted cars that are running lean burns out the valves had one with only 1/4 of the surface left. Only other damage to a valve can be from a broken belt so piston hits the valve or very rare the camshaft to snap which would cause the same thing.

[GavinPearson]

A photo would be useful.

It could be that if the valves had a welded face to protect for wear that there was residual stress present in the valve, resulting in an overload when hot, cracking and then quartile failure.

[PeterBurgess]

On a handful of occasions we have seen exhaust valve failure due to flaws in either the design where an abrupt change of curve leaves a tell tale line and the failure emanates from that line.We have seen production flaws allow failures too. The cracks always go at 90 degrees from where they start which is a good clue as to failure. If the engine is used with the failed valve then the damaged area can be smoothed by passing ex gases/combustion process and disguise the failure area.
More often we see ex failure due to oil contamination, this allows the ex valve to overheat and burn out.

Retarded ignition timing means the combustion process can still be well underway as the products pass the valve, this can overheat the valve and damage it. Remember the ex valve will grow longer due to heat and at some point may not have enough 'rest' time on the valve seat to allow sufficient cooling....all in all a viscious circle. The failure usually occurs in the hottest chamber (which is not always the hottest cylinder as well).

Peter

[smac]

[smac]

Hopefully these photos will load.

[Evoluzione]

smac said:

Hopefully these photos will load.

They have, sadly smaller than postage stamps so not of much use!

[GavinPearson]

Looks like a quartile failure to me.

With the valves still in the head, please remove the carbon from the other exhaust valves, inspect for cracks in the same general area as the broken one (i.e. around the circumference of the valve head) and report what you find. Thanks.

[PeterBurgess]

Hi Gavin

What is 'quartile failure'? I have only come across quartiles in statistical analysis when I was reading Psychology.

Peter

[Evoluzione]

It's where a quarter of the valve goes missing.

[PeterBurgess]

Thats exactly what my friend Dave Gollan said, I reckon you must have gone to the same University to do your engineering degree

[Evoluzione]

PeterBurgess said:

Thats exactly what my friend Dave Gollan said, I reckon you must have gone to the same University to do your engineering degree

Aye 'appen so. I wouldn't mind a trip down to yours one day, I think we might have some things in common - I'm not too far away in West Yorks.

[PeterBurgess]

You are most welcome to visit, the hot water is always on for tea or coffee.