Positive Crankcase Ventilation
WARNING: This page is about a different car, the 2002 Mercury Villager. However, it is still accessible from the selected car via links, so may be relevant.
The Positive Crankcase Ventilation (PCV) system uses intake manifold vacuum to extract blow-by vapors from crankcase. The mixture is then passed into the combustion chamber and burned. PCV valve provides control by metering flow of blow-by vapors according to manifold vacuum. PCV valve is located in valve cover.
Under conditions where abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), the system is designed to allow excess gases to flow back through crankcase vent hose and into air inlet for consumption during normal combustion.