PCV — Positive Crankcase Ventilation

During the power stroke, a small amount of combustion gas leaks past the piston rings into the crankcase below — this is called blow-by. Blow-by contains unburned hydrocarbons, water vapor, and acids. Left in the crankcase, these gases contaminate the oil, create sludge, and build pressure that pushes oil past seals. The PCV system vents these gases from the crankcase back into the intake manifold where the engine can burn them.

The PCV valve is a one-way flow control valve connected between the crankcase and the intake manifold. At idle — high manifold vacuum — the valve restricts flow to prevent pulling too much crankcase vapor. At cruise — moderate vacuum — the valve opens further to vent more gas. At wide open throttle — low vacuum — the valve allows maximum flow. The valve also prevents intake manifold backfire from reaching the crankcase.

A PCV valve stuck closed causes crankcase pressure to build. Oil gets pushed past seals and gaskets. Oil leaks appear at the valve cover gasket, front and rear crankshaft seals, and other sealing surfaces. A PCV valve stuck open causes a vacuum leak at idle — rough idle and lean fuel trims. A PCV system that is completely plugged forces blow-by gases to exit through the fresh air inlet hose instead — often pushing oil mist into the air intake and contaminating the MAF sensor.