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: Operation
With the engine operating, fresh air enters the PCV system through the air cleaner assembly. Fresh air flows through the crankcase breather and into the rocker arm cover compartment.
Entering fresh air combines with blow-by gases and unburned air/fuel mixture of the crankcase. Combined gases are drawn into the intake manifold, through the PCV valve, by manifold vacuum. Crankcase gases mix with air/fuel mixture and are burned in the combustion chamber. See Fig 1 .
The PCV valve is held closed by spring pressure when the engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, resulting in hard starting.
When the engine is running, manifold vacuum pulls the PCV valve open, allowing crankcase vapors to enter the intake manifold. A baffle in the rocker arm cover prevents engine oil from being drawn into the intake manifold.
If the engine backfires through the intake manifold, the PCV valve closes and prevents any flow of gases through it. This is to prevent the ignition of fumes in the 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.