A PCV valve, or Positive Crankshaft Ventilation valve, is a component of an internal combustion engine that recirculates the air and gasoline which has slipped by the piston uncombusted, collecting in the crankcase.
The uncombusted gasoline which collects in the crankcase can cause engine problems, and therefore the PCV valve was designed. When the pressure in the intake manifold is low, the gases from the crankcase are able to vent through the PCV valve back into the intake and on to the cylinders for another chance at combustion. When the engine speeds up and pressure in the intake manifold is higher than the crankcase, the PCV valve prevents the recirculation. This is the positive aspect of the valve, which only releases gases when the pressure is higher in the crankcase than in the intake manifold.