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.
Description & Operation
Crankcase ventilation systems prevent hydrocarbons from entering the atmosphere. This is accomplished by drawing vapors from the crankcase through a Positive Crankcase Ventilation (PCV) valve and routing them into the combustion chamber via the intake manifold. In the combustion chamber these vapors are burned in the combustion process. See Fig 1 .
The PCV valve is held closed by spring pressure when engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, a condition which could result in hard starting.
During engine operation, manifold vacuum pulls the valve open against spring pressure, permitting crankcase fumes to enter the intake manifold. Should the engine backfire, the PCV valve will close to prevent ignition of fumes in crankcase.
During certain engine operations, more blow-by is created than the PCV valve can handle. Excess blow-by is directed to the air cleaner and carburetor or Throttle Body through a breather assembly in the valve cover. It is then burned in the combustion chamber. The breather assembly acts as a separator to keep oil from being drawn into air cleaner during this operation.
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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.