PCV Valve (Positive Crankcase Ventilation) – (PCV) -FAQ

PCV Valve
PCV Valve

The PCV Valve (Positive Crankcase Ventilation) is a controlled device used to vent the crankcase. It sends partially burned gases that come from the engine’s crankcase back to the combustion chamber. Burning these gases for a second time is its main objective.

PCV Valve
PCV Valve

PCV – Stands for (Positive Crankcase Ventilation). One of the oldest and most used emission devices is the PCV valve. Although its name might sound very complicated, it is a very simple device. Overlooking this system is a common mistake as it is relatively simple and requires minimal maintenance.

The PCV valve is not only very useful but also has important benefits:

PCV Valve

  • Eliminates crankcase emissions.
  • Keeps the system free of moisture due to the constant air circulation that it implies.
  • Helps keep engine-damaging sludge to a minimum extending the life of your oil.
  • The PCV valve will also protect the engine in case of a backfire, which causes a sudden high-pressure pulse in the intake manifold. This forces the PCV valve closed so that the backfire flame can’t reach the crankcase, where it could ignite flammable fumes and cause damage.

It is critical that the PCV system must be kept clean. Otherwise airflow will be insufficient. A plugged or malfunctioning PCV system will eventually damage an engine. Contaminants accumulate in the engine oil, and unrelieved crankcase pressure can blow out gaskets and seals, creating oil leaks. A poorly maintained engine’s PCV system will eventually become contaminated with oil sludge.


The major component in the PCV system is the PCV valve. It consists of a simple spring-loaded valve with a sliding pintle inside. The movement of the pintle up and down changes the orifice opening to regulate the volume of air passing through the PCV valve.

The location of the valve allows it to pull vapors from inside the engine without sucking oil from the crankcase (baffles inside the valve cover or valley cover deflect and help separate droplets of oil from the blowby vapors).

A hose connects the top of the PCV valve to a vacuum port on the throttle body, carburetor or intake manifold. This allows the vapors to be siphoned directly into the engine.

Because the PCV system pulls air and blow by gases into the intake manifold, it has the same effect on the air/fuel mixture as a vacuum leak. This is compensated for by the calibration of the carburetor or fuel injection system. Consequently, the PCV system has no net effect on fuel economy, emissions or engine performance – provided everything is working correctly.



The flow rate of a PCV valve is calibrated for a specific engine application. For the system to function normally, therefore, the PCV valve must adjust the flow rate as operating conditions change. When the engine is off, the spring inside the valve pushes the pintle shut. This seals the crankcase and prevents the escape of any residual vapors into the atmosphere.

When the engine starts, vacuum in the intake manifold pulls on the pintle and sucks the PCV valve open. The pintle is pulled up against the spring and moves to its highest position. But the tapered shape of the pintle does not allow maximum flow in this position. Instead, it restricts flow so the engine will idle smoothly.

The same thing happens during deceleration when intake vacuum is high. The pintle is pulled all the way up to reduce flow. This will minimize the effect of blowby. When the engine is cruising under light load and at part throttle, there is less intake vacuum and less pull on the pintle. This allows the pintle to slide down to a mid-range position and allow more airflow.

Under high load or hard acceleration conditions, intake vacuum drops even more, allowing the spring inside the PCV valve to push the pintle valve even lower to its maximum flow position. What if blowby pressure builds up faster than the PCV system can handle it? The excess pressure flows back through the breather hose to the air cleaner. Now it is sucked back into the engine and burned.


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). 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 “unmetered” air to enter the engine and upset the fuel mixture, especially at idle where the idle mixture is most sensitive to vacuum leaks.

A plugged up PCV valve seems to be the most common problem. 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 valve. A restricted or clogged PCV valve cannot pull moisture and blowby vapors out of the crankcase. This can cause engine-damaging sludge to form, and a backup of pressure that may force oil to leak past gaskets and seals. The loss of airflow through the valve can also cause the air/fuel mixture to run richer than normal, increasing fuel consumption and emissions. The same thing can happen if the pintle inside the PCV valve sticks shut.

On late model vehicles with computer engine controls, the engine management system will detect any changes in the air/fuel mixture and compensate by increasing or decreasing short term and long term fuel trim (STFT and LTFT). Small corrections cause no problems, but large corrections (more than 10 to 15 points negative or positive) will typically set a lean or rich DTC and turn on the MIL.

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 (same diameter and hose fittings) 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.

Watch out for cheap replacement PCV valves. They may not flow the same as the OEM PCV valve.

Testing PCV Valve
Testing PCV Valve


1. Remove the valve and shake it. If it rattles, it means the pintle inside is not stuck and the valve should flow air.

2. Check for vacuum by holding your finger over the end of the valve while the engine is idling. This test tells you if vacuum is reaching the valve, but not if the valve is flowing properly. If you don’t feel vacuum, it means the valve or hose is clogged and needs to be replaced.

3. Use a flow tester to check the performance of the valve. This method is the best because it tests both vacuum and air flow.


The volume of air that is pulled from the crankcase by the PCV system is very important. This is because it takes a certain amount of airflow to remove the blowby vapors and moisture. But too much airflow can upset the air/fuel mixture in the engine.

So to check airflow, you can do any of the following:

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 (or you can disconnect the idle speed control motor so it won’t affect idle speed during this test). 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. Check the part number on the PCV valve to see if it is the correct one for the engine. The wrong valve may flow too much air. If there is no part number, replace the valve with a new one (which meets OEM specifications) and test again.

Measure the amount of vacuum in the crankcase.

vacuum guage
vacuum gauge

With the engine at normal operating temperature, block off the PCV breather tube or vent to the engine (usually the hose that runs from the air cleaner housing to the valve cover on 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. If you see a significantly higher vacuum reading, the intake manifold gasket is probably leaking and pulling vacuum on the crankcase (replace the leaky intake manifold gasket). If you see no vacuum, or find a buildup of pressure in the crankcase, the PCV system is plugged. Or it may not be pulling enough air through the crankcase to get rid of the blowby vapors.

NOTE: 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).

A smoke machine also works great for finding crankcase leaks as well as vacuum leaks. A smoke machine generates a smoke-like vapor by heating mineral oil. The mist can then fed into the intake manifold to check for intake manifold vacuum leaks, or into the crankcase to check for internal engine air leaks. Any leaks will allow the smoke to escape and you will see the smoke on the outside of the engine.


We see varied symptoms from a bad PCV valve, depending on the way it fails. For instance, a PCV valve that sticks open may cause a check engine light. The diagnostic trouble codes or DTC will normally not mention the PCV valve. Instead, the diagnostic trouble code P0171 and P0174 are common. These codes suggest a lean-condition in the engine. Other engines could set a mass-air-flow meter code or even an oxygen sensor code.

A failed PCV valve may also cause noise. Some will produce a whistle or whine and others can produce a low moaning noise. The easiest way to verify the problem is to temporarily block the vacuum source to the PCV valve and see if the noise changes or goes away. On some vehicles, a bad PCV valve can cause oil to blow onto the air filter element. An oily or dirty spot, near the PCV inlet hose is a symptom.


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