A closed crankcase ventilation system provides a more complete scavenging of crankcase vapors. Fresh air from the engine air induction system is supplied to the crankcase, mixed with blow-by gases and then passed through the crankcase ventilation housing into the intake manifold.

The primary control is through the crankcase ventilation housing which meters the flow at a rate depending on the manifold vacuum. In order to maintain the idle quality, the crankcase ventilation housing restricts the flow when the intake manifold vacuum is high. If abnormal operating conditions arise, the system is designed to allow excessive amounts of blow-by gases to back flow through the crankcase vent tube into the engine air inlet to be consumed by normal combustion.