Positive Crankcase Ventilation: Description and Operation

In gasoline engines, small amounts of combustion gases seep past the piston rings into the crankcase. These crankcase blow-by gases contain undesirable hydrocarbon air pollutants. To prevent these vapors from escaping into the atmosphere, while allowing proper ventilation of the crankcase the, Positive Crankcase Ventilation (PCV) system in used.

A positive flow of ventilation is routed through the crankcase by the system and the airflow is metered through the PCV vent housing according to engine needs.

Clean air downstream of the air filter is admitted into the engine crankcase through the throttle body. This air mixes with the gases in the crankcase and is then metered back into the intake manifold via the PCV hose on throttle body. The PCV vent housing removes oil vapors from the air flow before it enters the intake manifold.

Eventually, under normal operating conditions, sludge from crankcase vapors may clog the PCV system (adverse operating conditions like short trips in cold weather will speed up this sludge build up). When this occurs, the PCV system will no longer draw pollutants (unburned hydrocarbons) from the engine crankcase. These pollutants are then forced up into the air cleaner through the PCV breather tube, but without fresh air to dilute and help remove them, some remain in the crankcase. In time, this causes condensation of gases in the crankcase. This results in the formation of acids, sludge build-up and oil dilution.