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I wonder if oil leaks in diesels are caused by positive crankcase pressure. I believe this is a significant difference from ignition engines, which use manifold vacuum to suck out blow-by gasses. Diesels have no such vacuum and therefore blow-by gasses may find their way out every crevice possible, not only out the vent tube. Oil helps seal around gaskets and the positive pressure slowly pushes the oil out. This would be why oil can be found around the filler cap, vent tube, valve cover gasket, etc. So if a suction of some sort where placed on the vent tube (perhaps a small electric fan) might the leaking and mess be significantly reduced?
 

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My Father had an old 1966 Plymoth Valiant Station Wagon.

It had so much Oil Fill/Crankcase pressure that Oil would come out from between the Oil Pan Gasket.
The reason was that his Exhaust Valve Guides/Stems were badly worn.

Driveing with out the Crank Case Breather Cap solved the Oil leak issue but brought tears to your Eyes.
So gassers can also have high crancase pressure.

My 123 at over 200,000 miles does not leak any more oil than a comparable gasoline car.
But, I have fixed the leaks on it.
When I first bought it the Valve Cover was leaking badly.
I changed the Oil Filter to Block Gasket and the Fuel Injection to Block Gasket.
My Turbo Draing Grommet was also a source of leaks.
So none of the above stuff leaks now.

I still have a slight leak at the Front Crank Seal and maybe the Rear Main Seal but that is not uncommon for a 25 year and older car.

And yes, I do have some Blow By.
 
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