The crankcase on a detroit diesel is not pressurized by the blower. There is a 'air box' that surrounds the cylinders and the air is forced into the ports from this which is not part of the crankcase.

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Mark
82 300SD 110k
91 Caprice SS
92 Jetta TD
97 Cadillac Concours(300hp)
84 Celebrity 4.3L diesel