Crankcase Ventilation Systems: Operation
On gasoline models, intake manifold vacuum at idle pulls blow-by vapors through the oil screen, calibrated jet, and into intake manifold. The calibrated jet stabilizes ventilation flow at a rate that will not affect 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 vapors into air cleaner and through engine.
505 models shown; others similar.