Crankcase Ventilation Systems: Operation
On gasoline models, intake manifold vacuum at idle pulls blow-by vapors through the oil screen, calibrated jet, and into the intake manifold. The calibrated jet stabilizes ventilation flow at a rate that will not affect the idle mixture. When engine speed and load are high, blow-by exceeds the capacity of the calibrated jet. Intake manifold vacuum is low, but air cleaner vacuum is high enough to pull the vapors into the air cleaner and through the engine.