Very low compression in cylinder 2

[upallnight]

Back in the old days BC *(Before Computers) the only way to burn a valve was to run too much advance on the timing or run too lean of a mixture or both.

With Computer controlling everything from fuel mixture to ignition timing I find it hard to believe that you could burn a valve, without the computer warning you of the impending disaster. Hell if you don't put the gas cap on right you will set off a CEL for the evap system. Even a leak on the DISA valve o-ring will cause the computer to throw up a P0171 or P0174 DTC for a lean condition in bank one and bank two.

The two codes you posted are for misfires in cylinders. A lean condition is consider a misfire by the computer because the rotation speed of the crankshaft is not as great compared to a good cylinder.

I think the moral of this story is that one should not ignore CEL or SES even if you think the engine is running right because you are can not detect the little nuances like a computer.

[upallnight]

Originally Posted by DanSanDiego

Could just a leaking CCV cause the same symptoms (misfire in two cylinders with fuel shutoff)? Because maybe that is all I have wrong. The CEL light was never on previously and the O2 sensors were replaced about 6 months ago.

A leaking CCV is basically a vacuum leak. With a vacuum leak the cylinder will receive a weaker mixture. The weaker mixture will not create as much power as a cylinder with a normal or rich mixture. That is why the computer will assume that it is a misfire. The fuel shutoff is just a way for the computer to prevent damage to the cat converter. Even though the fuel is off, the computer will still provide a signal to the coils to fire the plugs. With no fuel available to cool the cylinder, the heat from the spark plug might had been enough to burn the valve.

Why was the 02 sensor replaced? You need to look at fuel trim readings and 02 voltage readings before assuming 02 sensors are at fault.

[stackz]

why not just shop around getting quotes for a place that will do the labor if you supply the parts?

then just buy a good used cylinder head and the gaskets and do it that way? probably save about a grand at least. I looked on ebay and good tested heads with valves are selling for $500 on average.

[upallnight]

Originally Posted by trader4

There is no way the spark from a sparkplug is going to generate
heat to burn a valve. Let's stick to reality. With the fuel cut off to
the cylinder, the heat will be far less than if it were operating.

Then what is your theory on how the valve got burned?

Why do you have to replace spark plugs, because after a while the electrode is burnt away from the spark.

[upallnight]

To the original poster, what type of driver are you? Are you a conservative driver, or do you have redline fever?

Driving a car at or near redline is hard on the valves. The exhaust valves endures a lot of heat since after the cylinder is done with the power stroke, it must push the hot exhaust gas past the exhaust valve. High rpm does not allow the valve sufficient time to transfer the heat to the valve seat.

A lean mixture as I said before does not provide enough gas to cool the cylinder on the intake stroke so the cylinder is burning hotter.

[upallnight]

Originally Posted by trader4

I don't have a theory. I just know that a spark plug sparking in a
cylinder with fuel cut off, is incapable of generating any significant
heat, much less burning a valve.

Then what happens to the electrode on spark plug?

Edit added picture.

[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.