Crankcase emission control (Peugeot 405)

To reduce the emission of unburned hydrocarbons from the crankcase into the atmosphere, with a sealed engine, crankcase gases and oil vapor are drawn from the crankcase through a wire mesh oil separator into the engine air supply tract and are burned by the engine.

At high collector depression (idling, slowing down) gases will be sucked out of the crankcase. At low manifold depression (acceleration, wide open throttle) gases exit the crankcase at a relatively higher crankcase pressure, and also, if the engine is worn, high crankcase pressure (due to increased gas breakthrough) will cause the flow to return to the collector.