What made this valve fail?

[buzzer]

Daughters Scenic started to misfire the other day. Its been running fine, she went to the shops, and on the way back it went onto three cylinders. No compression on number 4, and when I put the airline down the plug hole you could hear air in the exhaust.

On stripping, here's what I found. Its done 80K, she has owned it from new... What do you think is the cause of the failure? I have bead blasted the valve to make the damage clearer...

[oakdale]

Brake servo diaphragms are prone to failure on these(which would cause a weak mixture), where about does the vacuum pipe go onto the inlet manifold?

[one eyed mick]

Debris under valve seat ,sticky valve , tight valve clearance could all be the problem

[buzzer]

oakdale said:

Brake servo diaphragms are prone to failure on these(which would cause a weak mixture), where about does the vacuum pipe go onto the inlet manifold?

No sign of a weak mixture... nice and even colour, no faults logged in the ECU... other exhaust valve on that cylinder OK

[buzzer]

one eyed mick said:

Debris under valve seat ,sticky valve , tight valve clearance could all be the problem

No sign of the valve sticking, stem and guides we clean and free... Its hydraulic lifter.

It does not look burnt out?

[oakdale]

The fact that only one exhaust valve is burnt out means nothing, once a valve starts to go it burns out quick style.

That cylinder appears to have a lot more deposits on it than the next one, does the car burn any oil?

[buzzer]

oakdale said:

The fact that only one exhaust valve is burnt out means nothing, once a valve starts to go it burns out quick style.

That cylinder appears to have a lot more deposits on it than the next one, does the car burn any oil?

It does burn a lot of oil actually... Just took the pistons out, bores have minimal wear, ( checked with a mercer gauge) and the ring gap when you pop one in the top of the bore is only a couple of thou bigger than the new set I am just about to put in...

The valve steam oil seals are very hard though, and a loose fit on the stems, so I guess it was pulling oil in through there..

Have had a look at the valve now under a microscope. Its looking like the one side of the gap has been eaten away much more than the other side, and there appears to be an inclusion in the metal. I suspect there was a crack in the valve. Its certainly not run long enough to have burnt out a piece that big...

Bad luck I think

[shoehorn]

A small crack on an exhaust valve will turn into what you have there very quickly,
Any protrusion or indeed a crack directly exposed to combustion temps will get extremely hot,that valve would have probably been glowing.
I have seen smart car engines especially the later mhd`s(micro hybrid dynamics)with burnt valves at less than 10,000 miles usually because of oiling.

[Evoluzione]

Are the valve guides excessively worn?

[buzzer]

Evoluzione said:

Are the valve guides excessively worn?

Nope, no wear at all, I would say they are Calsibro which hardly wears. there was also zero wear on the valve stems.

[Mr2Mike]

buzzer said:

It does burn a lot of oil actually... Just took the pistons out, bores have minimal wear, ( checked with a mercer gauge) and the ring gap when you pop one in the top of the bore is only a couple of thou bigger than the new set I am just about to put in...

The valve steam oil seals are very hard though, and a loose fit on the stems, so I guess it was pulling oil in through there..

Not necessarily, piston rings can have a small gap but still seal badly. Running a honing tool down the cylinders to remove any glaze wouldn't be a bad idea if you are fitting new rings.

[Evoluzione]

Is it a turbo engine? What state are the mating surfaces of the valve and seat like? any way you can check valve clearances are correct?

I know some of the newer VW stuff (Polos IIRC) are having problems with burned out valves, I think it's down to worn guides. I was chatting to my neighbour at work, he has one in to do, if I get chance I'll take a look when he's got it apart.

Edited by Evoluzione on Saturday 19th October 16:08

[buzzer]

Evoluzione said:

Is it a turbo engine? What state are the mating surfaces of the valve and seat like? any way you can check valve clearances are correct?

I know some of the newer VW stuff (Polos IIRC) are having problems with burned out valves, I think it's down to worn guides. I was chatting to my neighbour at work, he has one in to do, if I get chance I'll take a look when he's got it apart.

Edited by Evoluzione on Saturday 19th October 16:08

None Turbo. The seat was perfect after a quick grinding in with some grinding paste, not even in need of re-cutting. Clearances were OK I guess as they are hydraulic lifters.

A few people have looked at it now and everyone feels like it was a faulty valve, probably from manufacture.