One of our Members explained this to me.

With no place to go the Crankcase Pressure pushed you Vacuum Shutoff into the shutoff direction and it reacts just as if you applied Vacuum to the opposite side of the Valve.

If I plug of the hole in my Valve Cover it takes about 7 seconds before the Engine starts to stumble and act like it is going to shut down.

High Crankcase pressure can also cause Oil top leak out of leaks more and past Seals.

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84 300D, 82 Volvo 244Gl Diesel