A crankcase ventilation system removes unwanted gases from the crankcase of an internal combustion engine. The system usually consists of a tube, a one-way valve and a vacuum source, such as the inlet manifold.

The unwanted gases, called “blow-by,” are gases from the combustion chamber that have leaked past the piston rings. Early engines released these gases to the atmosphere simply by leaking them through the crankcase seals. Positive crankcase ventilation systems send the crankcase gases back to the combustion chamber, as part of the vehicle emissions control, to reduce air pollution.