Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
PCV System - Gasoline: Operation
With engine operating, fresh air enters positive crankcase ventilation system through air cleaner assembly. Fresh air flows through crankcase breather and into rocker arm/valve compartment.
Entering fresh air combines with blow-by gases and unburned air/fuel mixture of crankcase. Combined gases are drawn into carburetor, through PCV valve, by manifold vacuum. Crankcase gases mix with air/fuel mixture and are burned in the combustion chamber. See Fig 1 .
PCV valve is held closed by spring pressure when engine is not running. This prevents hydrocarbon fumes from collecting in intake manifold, resulting in hard starting. See Fig 2 .
When engine is running, manifold vacuum pulls PCV valve open allowing crankcase fumes to enter intake manifold. A baffle in rocker arm cover prevents engine oil from being drawn into intake manifold.
If engine backfires through intake manifold, PCV valve closes and prevents any flow of gases through it. This is to prevent ignition of fumes in crankcase.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.