Very low compression in cylinder 2

[upallnight]

Originally Posted by trader4

Not much on mine, how about yours? I replaced original plugs at 125K, they were still fine, not badly eroded. Car was running fine, MPG before and after was the same. And what does what's happening at the actual
electrode, where it's exposed to plasma, have to do with the spark
somehow burning the exhaust valve, which isn't at the spark?

Depends on where the valve is burnt. The head is a hemispherical head so the spark plug is right in the middle of the four valves.

[DSSA]

The spark from a spark plug is very hot at the grounding point, but also very minuscule. By itself, it would have no chance of affecting a valve or valve seat.

A burnt valve is typically caused by excessive carbon buildup, incorrect ignition timing, incorrect valve lash, or poor AFRs.

I haven't pulled a ton of BMW heads, but the ones I have always surprise me with how much carbon buildup they have around the bases of the stems. I would venture a guess that some of them were due to the crappy valve seals BMW chose.

If your mechanic is correct in it being a burnt valve, you would need that head rebuilt. yes, "technically" you could fix just that valve/seat but the cost of just doing the whole head vs. the time/cost to R&R the head would make it prudent to have everything gone through. But just as importantly, I would want the cause to be tracked down.

Was it carbon buildup? A poorly flowing injector? Incorrect valve clearance? Etc..

I would also ask them how they diagnosed it being a burnt valve/seat.

[upallnight]

Originally Posted by DSSA

The spark from a spark plug is very hot at the grounding point, but also very minuscule. By itself, it would have no chance of affecting a valve or valve seat.

A burnt valve is typically caused by excessive carbon buildup, incorrect ignition timing, incorrect valve lash, or poor AFRs.

I haven't pulled a ton of BMW heads, but the ones I have always surprise me with how much carbon buildup they have around the bases of the stems. I would venture a guess that some of them were due to the crappy valve seals BMW chose.

If your mechanic is correct in it being a burnt valve, you would need that head rebuilt. yes, "technically" you could fix just that valve/seat but the cost of just doing the whole head vs. the time/cost to R&R the head would make it prudent to have everything gone through. But just as importantly, I would want the cause to be tracked down.

Was it carbon buildup? A poorly flowing injector? Incorrect valve clearance? Etc..

I would also ask them how they diagnosed it being a burnt valve/seat.

Wouldn't a leak in the CCV constitute a poor AFR? Extra unmetered air is introduced into intake manifold creating a lean condition. Carbon build up could have caused the valve to not seat properly thus overheating the valve and burning the valve.