What is the pressure at which crankcase relief door will open? The crankcase relief door, also known as the crankcase breather or ventilation valve, serves several purposes in a marine diesel engine: 1. *Pressure relief*: Allows excess pressure in the crankcase to escape, preventing damage from excessive pressure buildup. The crankcase relief door is an essential component of a marine diesel engine's ventilation system, ensuring proper engine operation, reducing wear and tear, and preventing dam