PCV stands for Positive Crankcase Ventilation. Unburned fuel vapors, other combustion by-products, and moisture are contained in the crankcase until they can be filtered and redistributed back into the intake manifold to be reburned by the engine. The main components of this system include a valve, hoses, and an air filter. The PCV valve restricts the amount of air that's siphoned from the crankcase into the intake manifold. The PCV filter introduces clean air through a hose to the engine.