Positive Crankcase Ventalation
The PCV or Positive Crankcase Ventilation system removes piston blowby from the crankcase. It relies on controlled flow to route these fumes into the intake manifold for combustion.
The system contains a PCV valve to control flow and a vent or breather tube to supply filtered air to the crankcase. It's mounted on the valve cover and sometimes the intake manifold. The valve opens during acceleration and cruise, allowing the engine's vacuum to draw the fumes into the intake manifold. The vent tube provides fresh air to the crankcase.
A stuck open PCV valve results in a massive vacuum leak and a rough idle. A stuck closed PCV valve results in excessive crankcase pressure and oil forced past seals and gaskets. Engine breather hoses and filters get kinked or clogged, restricting flow and allowing blowby gases to build in the crankcase.