What is the PCV System?

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Positive Crankcase Ventilation (PCV):

The Positive Crankcase Ventilation (PCV) system reduces blowby emissions from the engine. About 20% of the whole hydrocarbon (HC) emissions produced by a vehicle are blowby emissions from gases that get past the piston rings and enter the crankcase. The upper the mileage on the engine and also the greater the wear and tear on the piston rings and cylinders, the greater the blowby into the crankcase.

Before PCV was invented, blowby vapors were simply vented to the atmosphere through a “road draft tube” that ran from a vent hole in a very valve cover or valley cover down toward the bottom.

In 1961, the primary PCV systems appeared on California cars. The PCV system used an intake vacuum to siphon blowby vapors back to the manifold. This allowed the HC to be re-burned and eliminated blowby vapors as a source of pollution.


The system proved to be so effective that “open” PCV systems were added to most cars nationwide in 1963. An open PCV system draws air in through a mesh filter inside the oil filler cap or a breather on a valve cover. The flow of fresh air through the crankcase helped pull moisture out of the oil to extended oil life and reduce sludge. The sole drawback to those early open PCV systems was that blowby vapors could still backup at high engine speed and loads, and escape into the atmosphere through the oil filler cap or valve cover breather.

In 1968, “closed” PCV systems were added to most cars. The breather inlet was relocated inside the filter housing so if pressure ensured it’d overflow into the air filter and be sucked down the carburetor. No vapors would escape into the atmosphere.

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