System Operation

As an engine runs, high-pressure gases are contained within the combustion chamber. Piston rings seal against the cylinder preventing the high pressure gases from passing into the crankcase between the side of the piston and the cylinder bore. Some amount of gas always leaks past the piston rings into the crankcase. If this blow-by gas could not escape then pressure would build up within the crankcase. Should the intake manifold's pressure be greater than the crankcase the PCV valve closes to prevent reversal of the exhausted air back into the crankcase again.

It is critical that the parts of the PCV system be kept clean and open, otherwise air flow will be insufficient. A plugged or malfunctioning PCV system eventually damages an engine. An engine's poorly maintained PCV system eventually becomes contaminated with sludge, causing serious engine problems.