The Signs of a Faulty PCV Valve

Updated February 21, 2017

The PCV (positive crankcase ventilation) valve connects the combustion chamber to the crankcase. When the gases go back to the engine, they are burnt again, increasing the vehicle's fuel economy. The main function of the valve is to control the movement of the partially burnt gases in the crankcase to the combustion chamber. This helps to extend the life of the engine, as well as keeping moisture out of the fuel system.


When the valve breaks, vapours will not be removed from the engine. This results in pressure build-up in the crankcase.


When the pressure in the crankcase gets really high, it will create a backward flow of air. This causes oil at the PCV entrance to be pushed back with the air and get in the air filter. Oil will also run through the intake tube and flow down to the throttle body. The throttle body will then be faulty and cause the accelerator to stick.

Rough Idle

If the valve becomes stuck open, there is too much air flow and vapour going to the engine during idle, when the valve should be closed. This results in rough idling and can cause the engine to stall.

Lack of Rattle

If you have access to the PCV valve, take the valve and give it a good shake. A PCV valve that works properly will rattle. One that is stuck will not rattle.

Weak Vacuum

When the engine is running, place your finger at the valve end. Normally there will be a strong vacuum. If there is no vacuum, the valve tube has a leak or is clogged.

Fuel Economy Decrease

If the valve is leaky, there is less air in the fuel system. This means the fuel mixture changes and the engine will produce less power with each tank of gas.

Blue Smoke

Blue smoke from the tailpipe occurs when the valve is clogged and oil drips into the intake manifold. If oil is pushed past the oil seals in the valve, this can produce blue smoke.

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About the Author

Liz Tomas began writing professionally in 2004. Her work has appeared in the "American Journal of Enology and Viticulture," "BMC Genomics" and "PLoS Biology." She holds a Master of Science in food science from Cornell University and a Bachelor of Science in biochemistry from the University of New Hampshire. She is pursuing her Ph.D. in oenology at Lincoln University.