Positive Crankcase Ventilation (PCV)
WARNING: This page is about a different car, the 1995 Ford RV Cutaway, 1995 Ford Econoline, 1995 Ford Cutaway, and 1995 Ford Club Wagon. However, it is still accessible from the selected car via links, so may be relevant.
The 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 amount 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.