I have had a problem with oil draining out my wastegate drain that has been driving me nuts. I too have excessive amount of oil in my air filter housing.

Here is where it has led me:
Today I used a bypass cyclone to run as an oil seperator and frankly it worked terribly, but that is not the point, I completely disconnected the breather tube from the oil seperator to isolate if my oil is coming from teh breather or the turbo.

What I found is that the turbo could not be producing the oil and neither is the blowby (which was my main concern). Now I am not completely convinced that II have found the problem but what I did was
I eliminated the turbo since I have oil in the throat of the intake at the air filter. I don't think the turbo can throw oil back up into the air cleaner housing.
So... I pullled the top off the seperator and completely cleaned it out using a rag. I also cleaned all the oil out of the air filter and the throat of the intake.

I then replaced the top of the oil seperator and drove the car the car hard on a hill climb for about ten minutes.

Low and behold the air filter had fresh oil on the surfaces. I then pulled the top off the seperator and it is covered with oil. No possible connection to the crankcase vent system.

I am thinking that my chekc valve is bad although I have not tested it yet but by the manual it states that the check valve was put into place to stop the system from withdrawing the oil vapors from the pan.

I will test mine tomorrow and let you know my results. What bothers me is I cannot find any info on the replacement of this check valve and by the pictures it indicates that it is in the pper pan but possibly mounted romt he inside. Could be trouble...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique