Hissing from Oil Dipstick

Vacuum in the crankcase can actually be good. In the racing community we try to put a vacuum in our crankcase on purpose. I have a pan-evac system on my drag car that works off of the headers to "pull" a vacuum in the crankcase. The best systems are belt driven vacuum pumps for this purpose. Rings seal better when there is a vacuum present in the crankcase.

The negative to having a vacuum in the crankcase is it will try to pull dirt and moisture in if the motor is not sealed well or if the vacuum overcomes seals or gaskets.

Chuck