PCV Valve


A rough-idling engine can signal a number of positive crankcase ventilation (PCV) valve problems, such as a clogged valve or a plugged hose. But before beginning the functional checks, double check the PCV valve part number to make certain the correct valve is installed. If the correct valve is being used, continue by disconnecting the PCV valve from the valve cover, intake manifold, or hose. Start the engine and let it run at idle. If the PCV valve is not clogged, a hissing is heard as air passes through the valve. Place a finger over the end of the valve to check for vacuum.

With the engine at idle, vacuum should be felt at the PCV valve.

If there is little or no vacuum at the valve, check for a plugged or restricted hose. Replace any plugged or deteriorated hoses. Turn off the engine and remove the PCV valve. Shake the valve and listen for the rattle of the check needle inside the valve. If the valve does not rattle, replace it.

Some vehicle manufacturers recommend that the valve be checked by removing it from the valve cover and hose. Connect a hose to the inlet side of the PCV valve, and blow air through the valve with your mouth while holding your finger near the valve outlet.

Blowing through the inlet of a PCV valve should cause air to freely flow through. Reprinted with permission.

Air should pass freely through the valve. If air does not pass freely through the valve, replace the valve. Move the hose to the outlet side of the PCV valve and try to blow back through the valve.

Blowing through the outlet of a PCV valve should cause air to barely flow through. Reprinted with permission.

It should be difficult to blow air through the PCV valve in this direction. When air passes easily through the valve, replace the valve.

Do not attempt to suck through a PCV valve with your mouth. Sludge and other deposits inside the valve are harmful to the human body.

Another simple check of the PCV valve can be made by pinching the hose between the valve and the intake manifold with the engine at idle.

When the PCV hose is pinched, the valve should click. Courtesy of American Honda Motor Company, Inc.

You should hear a clicking sound from the valve when the hose is pinched and unpinched. If no clicking sound is heard, check the PCV valve grommet for cracks or damage. If the grommet is all right, replace the PCV valve.

Remember that proper operation of the PCV system depends on a sealed engine. The crankcase is sealed by the dipstick, valve cover, gaskets, and sealed filler cap. If oil sludging or dilution is found and the PCV system is functioning properly, check the engine for oil leaks and correct them to ensure that the PCV system can function as intended. Also, be aware of the fact that an excessively worn engine may have more blowby than the PCV system can handle. If there are symptoms that indicate the PCV system is plugged (oil in air cleaner, saturated crankcase filter, and so forth) but no restrictions are found, check the wear of the engine.