2006 MINI Cooper burnt exhaust valve

[Richyvrlimited]

Any ideas what may have caused this? It's from my brothers R50 MINI Cooper. He bought it from my mother, who owned from new. It's done 80k

We knew there was low compression on cylinder number 4, a wet compression test showed the rings were fine, so pulled the head and this is what we found.





It was the outermost exhaust valve, all the others looked absolutely perfect. The engine hasn't ingsted anything, the pistons cleaned up nicely, and you can still see nice even cross hatching on the bores, no abnormal wear at all.

Other than a missfire at low revs/idle the car was apparently fine to drive, and with the valve fixed it drives like new. It's followed the BMW service schedule until it came out of warranty, and then we service it ourselves (Oil 12 months/6k whichever comes first etc etc).

[Richyvrlimited]

It has HLAs none were sticking.

Miss fire was definitely caused by the low compression, not the other way around.

How would oil type make a difference? Wouldn't poor fuel cause all the valves to fail similarly? I suppose it could just be a weak valve.

[Richyvrlimited]

Evoluzione said:

Did you check for valve guide wear?

No I didn't even know I could/should! Please Excuse the obtuse question, but how would I do that?

[Richyvrlimited]

Pumaracing said:

This is a very typical high temperature fatigue failure caused by the valve cracking in the heavily loaded seat region and then losing material due to gas blowby and erosion. The Mini engine has very small exhaust valves as a proportion of the inlet valve size and power output and they have a hard life. It's possible that a miniscule material defect during forging initiated the crack in that particular valve and from then on failure is just a matter of time. It's unlikely that this failure points to a defect in any other part of the valve train but it's always sensible to check for perfect valve seating either by light lapping in with fine paste, examining with engineer's blue or with a 1 thou feeler gauge and test the drag all round the valve seat. If the seat in the head is not concentric the feeler gauge won't grip evenly all round.

Ahh brilliant, thanks for the detailed response.

I lapped in the new valve briefly with a coarse lapping compound and then finished off with fine, so hopefully that will have sorted the valve seating out.

Many thanks for the other tips, I'll remember them for if there is a next time!

[Richyvrlimited]

Pumaracing said:

Hmmm. You should read this.

http://www.pistonheads.com/gassing/topic.asp?t=980...

I used the coarse lapping compound for about 5 seconds maximum, and with very light pressure on the valve.

Then I moved to the fine and lapped for about 10 - 15 seconds. hopefully I didn't overdo it. Too late now either way though...

[Richyvrlimited]

one eyed mick said:

20 secs of lapping ? you might aswell not bother, after about 45 years of experience that is bull pucky, do the job properly !!!,if new valves and seats you should not need coarse paste , if seats and valves have not been surfaced you will need minutes of coarse and fine to get any thing like a seat !!!!

Have you read the post from Pumaracing?

"However a very light lap with fine paste for just 10 seconds or so to check that the valve and seat are truly concentric and with no high or low spots is a good idea and not a problem. If there isn't an even grey contact area all round both valve and head seat after that then it's probably time for remedial machining rather than further lapping."

The car runs well now with good even compression numbers across the board, so i'm not too concerned. I'm happy with the failure reason Pumaracing states. If it lasts another 80k then it was a weekend and a few ££ well spent.