Piston Engines

The PCV system includes 2 hoses and a PCV valve. Air normally flows out of the valve cover hose, through the PCV valve and into combustion chambers. A hose from the air cleaner to the valve cover supplies fresh air to the crankcase.

When intake manifold vacuum drops due to heavy loads, the amount of blow-by exceeds the capacity of the PCV valve. Air then flows directly from the valve cover into the air cleaner and carburetor. When engine is not running, the PCV valve is closed and vapors are stored in the crankcase.