PCV System – One Of The Biggest Causes Of Engine Oil Sludge

PCV System
PCV System

Did you know the most common problem in the PCV system is a plugged up PCV valve.

Another problem is a lack of air flow to carry vapors from the crankcase.

Without air flow, moisture contamination remains and a sludge buildup is often a result.

Operating the engine without adequate ventilation is a leading cause of engine sludge.

An accumulation of fuel and oil varnish deposits and/or sludge inside the valve can restrict or even block the flow of vapors through the PCV system.

A restricted or plugged PCV system cannot pull moisture and blowby vapors out of the crankcase.

This can cause engine-damaging sludge to form.

Also,  A backup of pressure that may force oil to leak past gaskets and seals.

The loss of airflow through the PCV system can also cause the air/fuel mixture to run richer than normal. The same thing can happen if the pintle inside the valve sticks shut.

If the pintle inside the PCV valve sticks open, or the spring breaks, the PCV valve may flow too much air and lean out the idle mixture. This may cause a rough idle, hard starting and/or lean misfire (which increases emissions and wastes fuel).

Loose Or Leaky Hoses

Leaking Hose
Leaking Hose

The same thing can happen if the hose that connects the valve to the throttle body, carburetor or intake manifold pulls loose, cracks, or leaks. A loose or leaky hose allows “un-metered” air to enter the engine and upset the fuel mixture, especially at idle where the idle mixture is most sensitive to vacuum leaks.

Incorrect Valves

Are The Valves The Same
Are The Valves The Same

Problems can also occur if someone installs the wrong PCV valve for the application.

The flow rate of the PCV valve is calibrated for a specific engine application.

Two valves that appear to be identical on the outside may have different pintle valves and springs inside, giving them very different flow rates.

A PCV valve that flows too much air will lean the air/fuel mixture, while one that flows too little will richen the mixture and increase the risk of sludge buildup in the crankcase.

Basic Airflow Testing

Pinch or block off the vacuum hose to the PCV valve with the engine idling at operating temperature. The engine idle rpm should typically drop about 50 to 80 rpm before the idle speed corrects itself. If there is no change in idle speed, check the PCV valve, hose and breather tube for a restriction or blockage. A greater change would indicate too much airflow through the PCV valve. The wrong valve may flow too much air.

Checking For Leaks

Vacuum-Pressure Pump
Vacuum-Pressure Pump

Measure the amount of vacuum in the crankcase. With the engine at normal operating temperature, block off the PCV breather tube or vent to the engine. Pull out the dipstick and connect a vacuum-pressure gauge to the dipstick tube. A typical PCV system should be pulling about 1 to 3 inches of vacuum in the crankcase at idle. A significantly higher vacuum reading, the intake manifold gasket is probably leaking and pulling vacuum on the crankcase. If you see no vacuum, or find a buildup of pressure in the crankcase, the PCV system is plugged or is not pulling enough air through the crankcase to get rid of the blowby vapors.

If the engine has a leaky oil pan, valve cover or intake manifold gasket leak, or leaky crankshaft seals, it will not be able to develop much vacuum in the crankcase because it is pulling in outside air (which is also unfiltered and can further contaminate the oil).

To find a crankcase air leak, you can lightly pressurize (no more than 1 to 3 psi) the crankcase with shop air via the dipstick tube or oil filler cap or breather after blocking all the other vents. Do not use any more air pressure than this or you may create leaks where there were no leaks before. Then use a spray bottle to squirt soapy water around the gasket seams and seals. If you see bubbles, you have found an air leak (replace the gasket or seal as needed and retest).


Keep in mind that some new models don’t have a valve at all; instead, you’ll find a simple vacuum hose going from the valve cover to an air inlet duct. Others may have a simple restrictor in place. Still, you can check the restrictor, hoses and other components.

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