by 
The Curious Bag
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Known properly as a "PCV", the abbreviation for Positive Crankcase Ventilation is often mistakenly called a "PVC." These systems allow built-up crankcase pressure to release and have been used nearly as long as internal combustion engines have existed.
Crankcase Pressure
The hot gases that escape past an engine's piston rings wind up in the crank case. Without some way to vent this pressure, engine failure is an inevitable conclusion.
PCV Valve Function
The PCV valve is used to allow the pressurized crank-case air to exit, while keeping vital engine oil inside.
PCV Valve Location
These valves are usually located in one of the engine's valve covers and are connected to either the intake manifold or carburetor by a rubber hose.
Vacuum Clearing
The "positive" part of PCV is due to the fact that PCV-equipped engines use the suction of their combustion cycle to create a vacuum in the crankcase.
PCV Failure
If your PCV valve becomes blocked, pressure will build in the crankcase and hurt the motor. However, more often than not, extreme pressure build-up will clear any obstruction in the valve.
Source:
Resource:
Video: PCV Valve Replacement
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