Help me diagnose this oil leak...

[paanta]

I've got an engine oil leak that's getting worse on my '91 535i w/ 225K on the odo. It seems like a rear main seal issue. Oil visibly seeping out between the oil pan and bell housing, a bit oily inside the holes in the bell housing, some oil puddled right underneath the starter, etc. It doesn't leak enough that I have to top the oil off, but it's making a hell of a mess.

The previous owner already replaced a rear main seal and oil pan gasket, though. That has me a bit worried.

When the car is warm, in neutral, I've got a bit of a knock knock knock sound that comes and goes depending on engine speed. It goes away if I put the clutch in.

I don't see any movement on the end of the crank when I rev the car in neutral. Nor do I see much (any) in-out movement on the crank if I put a pry bar on the vibration damper (though the damper itself seems to flex a bit).

Any idea why a rear main seal would go out in well under 5K miles? Remedies?

[bsell]

Good start on the detective work!

If the PO didn't dress the groove in the crank that the seal rides in, the groove could have taken out the new seal...or the PO did a 'fail' job and cut the seal during install.

Since you've been so thorough with the diagnosing already, I hate to ask if you went north of the crank seal looking for the leak...this isn't a new leak at the back of the head/block junction is it?

[paanta]

I spent months trying to stop a valve cover leak that I was hoping was the source of all the oil. That leak is gone, and now it's clearly just seeping straight out between the engine and transmission, right at the bottom. The oil buildup outside the check holes is also getting worse.

I've heard the thrust bearing on the crank can cause a rattle and help to trash rear main seals. Where does that bearing live? Real OEM isn't real clear on which one actually controls the play in the crank.

[bsell]

Thrust bearing is #2 on this page:
http://www.realoem.com/bmw/showparts...74&hg=11&fg=20

I've seen an old Subaru w/mileage similar to yours where the crank would move at least 1/4 inch when the clutch was pushed in! I can say it surprised the hell out of me when the other mechanic stepped on the clutch and the balancer 'jumped' at me! That thing didn't leak at the main seals...weird.

Yes, I would figure you could see the crank moving back and forth as an assistant stepped on and released the clutch if your thrust bearing was done in. You would figure the front main seal would leak also due to the movement...

Is this the rear main seal? http://www.realoem.com/bmw/showparts...45&hg=11&fg=10

If so, could be the seal is solid but the gasket has given up the ghost.

[paanta]

Damn, I was hoping it was at one end or the other.

I assume there's no way to get at that thing with the engine in place, if it's the problem (which it doesn't seem to be).

Now the question is whether I wait until spring when the garage is a bit more pleasant, or fix it now before my clutch is _totally_ soaked in oil.

[paanta]

Or perhaps the PO bent the cover plate.

225K is low enough miles on an M30 that hopefully it was just a botched seal install. Especially since the engine runs and sounds totally fine. Plus, I've seen almost no posts on problems with the crank bearings or rear seal failure on these cars.

I just hate to pull the transmission, fix the seal, and then be right back doing more heavy lifting to swap the whole engine when the seal fails again.