After post a thread about "Lube oil leakage" I received many good posts and very helpful ideas about this issue. I found no leaks anywhere in the engine other than that area where engine and transmission are connected.
A friend suggested me to check for clog on some line or tube that release positive pressure from engine. He said that if the engine is running pressurized it will force the oil to leak from crankcase and in this case the rear seal once worn out will be the first place to have oil leakage. Today I try to see if it was happen and after start the car and leave the engine in idle I check for that pressure and I found out that removing the tube from the top of valve cover to air filter there are built pressure. I put it back in place on top of valve cover and then I disconnected the other end on top of air filter and using my thumb to plug it I also found the same pressure. It mean that the tube is not clogged but when I take it out from valve cover a kind of air blast pop out from there. If I hold that hole on valve cover plugged with my thumb the engine will stop running like closing the air intake in a gasoline car.
Now there is the question: It is normal for the engine work under that pressure? Should not the connection from bottom of air filter to engine let that pressure be released? I would like to hear what anyone has to say about it.
Thanks.

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Nyck - TX - USA

Engines normally produce some pressure in the crankcase. This comes from combustion gasses blowing by the piston rings into the crankcase, which is why it is called blowby. A certain amount of it is normal, and the function of the tube you were messing with is as you guessed-to vent that pressure and direct it into the intake tract so that it can be burned to reduce emissions. In the old days, the same tube would have been vented to the atomosphere and was called a draft tube.

The engine stall from plugging the tube tends to suggest that little if any blowby is making it out of the engine along any other path, though it is certainly still possible that you have a rear main seal leak.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Hi

You don't say what engine you have but the blowby pressure should be venting into the air filter/intake through the breather hose from the valve cover. If you have an oil seperator in your air filter it might be possible for it to be gunked up enough to restrict the flow of the blowby gases thereby not properly releasing the crankcase pressure.

What happens when you let the oil filler cap sit loose on the hole while the engine is idling? Does it just dance a little or blow off?

How does your air filter look?

Hi:
You brought an important issue even when I forgot to mention my engine.
The engine is a Mercedes Benz 300SD Diesel and the car is a 126 Chassis year 1982 with 144K miles. I hope this info can help.
I was really wondering about that oil separator inside of the air filter but as I never saw any comment about that device I did not pay much attention to it but I agree with you when you mentioned about the gunk on it.
Is that separator washable or is suppose be replaced when dirty or clogged?
My air filter is clean but the filter housing is a little wet of oil I believe because the gases from engine is trying to go back to intake and probably that separator is blocking it. I loose the oil filler it will dance and also blowby gases when idling. If I remove the engine oil dipstick blowby will come from there also.
Thanks for you help.

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Nyck - TX - USA

Hi:
Your post was very helpful to make me understand how that vent should work.
Based on your knowledge do you think if that vent is obstructed in somewhere it will force the pressure, generated in the engine crankcase, to escape through any other possible location of the engine? If so can the real seal be one of this location?
Another member on this forum mentioned about the oil separator inside of the air filter and the chance of it be clogged and not allowing gases to flow freely into the intake and automatically increasing that pressure from engine crankcase which make sense.
I will be glad if you can send me some more info about this issue.
Thanks for your help.

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Nyck - TX - USA