PCV System
WARNING: This page is about a different car, the 2009 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.
The PCV system uses intake manifold vacuum to ventilate blow-by fumes from the crankcase and return the fumes to the intake manifold for combustion. The 2.3L engine does this through a coolant-heated fitting. The PCV valve varies the amount of blow-by gasses returned to the intake manifold based on available engine vacuum. The PCV valve also prevents the entry of combustion backfiring into the crankcase.