Positive Crankcase Ventilation (PCV)
WARNING: This page is about a different car, the 1995 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.
PCV system uses intake manifold vacuum to recycle blow-by vapors from the crankcase to the combustion chamber, where they are burned. PCV valve meters flow of blow-by vapors, according to manifold vacuum.
When high amounts of blow-by gases are produced (such as worn piston rings), excess gases flow back through crankcase vent hose into the air inlet and are burned during normal combustion.