2006 MINI Cooper burnt exhaust valve
Discussion
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).
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).
Hydraulic or solid lifters ?
Check valve clearances, dont use crap fuel, and use correct oil.
On top of those...it does almost seem either fuel is getting worse...or some manufacturers are cheaping out on valve or valve seat material leading to recession...leading to..what you have there.
Check valve clearances, dont use crap fuel, and use correct oil.
On top of those...it does almost seem either fuel is getting worse...or some manufacturers are cheaping out on valve or valve seat material leading to recession...leading to..what you have there.
h to change so a lot of people don't bother, the plug wears to the point it doesn't work, a misfire develops, and the exhaust vales burn out due to temperature.Richyvrlimited said:
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.
I know in the past, some engines had sticky valves with the wrong oil, Ford a few years back ?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.
When parts fail, usually one fails first...multiple valves wont burn at exactly the same time.
Basically if the valve is held open slightly when it should be closed, or doesnt get enough seat time, it can burn the valve as the valve never gets a chance to cool down.
There could also be fuel/timing issues leading to higher combustion/EGT temps too, but you'd need to investigate that once running again.
And correct, I could see no possible way a misfire would ever cause that damage. But the valve would certainly lead to a misfire/poor running.
Were all the installed heights of the valves identical or near when you had the head stripped ? ie any signs of a problem, recession, wear etc ?
Lifters definitely not bottomed out or anything ?
stevieturbo said:
I know in the past, some engines had sticky valves with the wrong oil, Ford a few years back ?
When parts fail, usually one fails first...multiple valves wont burn at exactly the same time.
Basically if the valve is held open slightly when it should be closed, or doesnt get enough seat time, it can burn the valve as the valve never gets a chance to cool down.
There could also be fuel/timing issues leading to higher combustion/EGT temps too, but you'd need to investigate that once running again.
And correct, I could see no possible way a misfire would ever cause that damage. But the valve would certainly lead to a misfire/poor running.
Were all the installed heights of the valves identical or near when you had the head stripped ? ie any signs of a problem, recession, wear etc ?
Lifters definitely not bottomed out or anything ?
A misfire can cause valve failure, it will take a long time, but it can happen. Google Smart car burnt valves, what I hadn't taken into account when I typed the above was the Smart is 2v per cylinder, where is the Mini is 4v, also the failure is slightly different, a chunk of the valve goes missing, where as the failure picture almost looks like the seat area has split open.When parts fail, usually one fails first...multiple valves wont burn at exactly the same time.
Basically if the valve is held open slightly when it should be closed, or doesnt get enough seat time, it can burn the valve as the valve never gets a chance to cool down.
There could also be fuel/timing issues leading to higher combustion/EGT temps too, but you'd need to investigate that once running again.
And correct, I could see no possible way a misfire would ever cause that damage. But the valve would certainly lead to a misfire/poor running.
Were all the installed heights of the valves identical or near when you had the head stripped ? ie any signs of a problem, recession, wear etc ?
Lifters definitely not bottomed out or anything ?
It was Ford with the sticky valves due to oil spec, when they launched the Zetec engine, they specified a low viscosity oil, 5W-30 if memory serves, which was widely available at the time, so people ran them on what the had and sticky valves was the result.
Megaflow said:
A misfire can cause valve failure, it will take a long time, but it can happen. Google Smart car burnt valves, what I hadn't taken into account when I typed the above was the Smart is 2v per cylinder, where is the Mini is 4v, also the failure is slightly different, a chunk of the valve goes missing, where as the failure picture almost looks like the seat area has split open.
Burnt valves in Smarts are caused by poor ring sealing and the subsequent heavy oil consumption creating carbon deposits on the valve seats. Nothing to do with misfires from old spark plugs.Richyvrlimited said:
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?3 cylinder Polo engines are renowned for it.
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.
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 said:
Ahh brilliant, thanks for the detailed response.
I lapped in the new valve briefly with a coarse lapping compound ...
Hmmm. You should read this.I lapped in the new valve briefly with a coarse lapping compound ...
http://www.pistonheads.com/gassing/topic.asp?t=980...
Pumaracing said:
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...
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 !!!!
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.
Richyvrlimited said:
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...
You'll be absolutely fine at that. It takes a couple of minutes of determined effort to really damage a valve and seat like the CVH guy I mentioned used to do to every head I built for him.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...
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