PCV Valve

Countless explosions occur every minute your engine runs. While the good majority of this internal combustion is confined to the combustion chambers, a small amount of the controlled explosion sneaks by the piston rings and creates blow-by gases inside the engine crankcase. These combustion by-products must be vented away for proper engine operation.

Positive Ventilation
Back in the days of environmental abandon, engines vented any crankcase gases and blow-by materials through a tube right out onto the ground. No valves required. As time and engineering improvements marched on, a simple system was devised to re-burn these gases and materials back through the engine for cleaner air. At the heart of this system is the PCV valve.


PCV is an acronym for positive crankcase ventilation. A PCV valve is only half of a two-part system. At one end of the system is a breather, which allows fresh air into the crankcase. At the other end of the system is the PCV valve itself, which meters spent gases and combustion by-products back through the engine relative to engine demand.


At idle the engine isn't working very hard and the valve remains mostly closed, allowing a small amount of the crankcase gases back through the engine. At full throttle the engine works harder and creates more crankcase gases, so the PCV valve opens wide to let more through. If the valve gets gummed up with gunk it won't meter properly.

When to Replace
If the PCV valve is stuck open at engine idle, too great a quantity of spent gases are let through. Rough engine idling and stalling can be the result. If the PCV is stuck closed, pressure inside the crankcase can build up and the gases can force engine oil up through the breather and air filter causing excessive oil consumption and a fouled intake system. The good news is that replacing the PCV valve is both easy and relatively inexpensive.