Troubleshooting the PCV System
A PCV valve uses light spring pressure to hold the valve in the open position while the engine isn't running or when there's a drop in manifold vacuum, like during acceleration. This open valve results in maximum crankcase ventilation. When the engine is at idle or cruise, the intake manifold vacuum is high and pulls the valve upward, blocking the port to the intake manifold. At idle, manifold vacuum pressure is higher than the PCV valve's spring pressure.
A stuck closed PCV valve results in excessive crankcase pressure and oil leaking past oil seals and gaskets. They get clogged, and blowby gases build in the crankcase. Oil travels up the clean air hose and into the air filter resulting in a contaminated air filter or breather. Some air supply hoses have an air filter or filament to clean the air entering the crankcase.
A stuck open PCV valve or leaking PCV hose results in a lean air-fuel ratio and drivability issues like surging. When the valve is removed from the valve cover, vacuum should be present at its opening. When your thumb is placed over the hissing vacuum leak, the plunger should "snap back" into the valve. When removed from the engine, the PCV valve should rattle when shaken. Please note that if the valve does not rattle, it's clogged and must be replaced.