Crankcase emission control equipment is separated into two different systems: Positive Crankcase Ventilation (PVC) and Crankcase Ventilation System (CCV). The systems perform the same function, differing only in the way the exhaust gases are metered. The PVC system uses a valve, containing spring loaded plunger, which meters the amount of crankcase vapors routed to the combustion chamber based on manifold vacuum. The CCV system contains a metered orifice of a calibrated size which meters the amount of crankcase vapors drawn from the engine based on manifold When the engine is running, a small portion of the gases which are formed in the combustion chamber during combustion leak by the piston rings and enter the crankcase. Since these are under pressure, they tend to escape from the crankcase and enter into the atmosphere. If these gases were allowed to remain the the crankcase for any length of time, they would contaminate the engine oil and cause sludge to build up. If the gases are allowed to escape into the atmosphere, they would pollute the air, as they contain unburned hydrocarbons.

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