PCV System
WARNING: This page is about a different car, the 2010 Ford Focus. 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 vapors and evaporated oil from the crankcase and returns them to the intake manifold for combustion. The PCV valve varies the amount of blow-by vapors and evaporated oil returned to the intake manifold based on available engine vacuum.