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The positive crankcase ventilation or PCV valve is an inexpensive and often overlooked component. It is also one possible cause of expensive oil leaks and sludge buildup in an engine.

All automotive engines are lubricated with oil and when oil is churned by moving parts, pressure is produced. Piston rings and valve guides also leak slightly producing pressure in the crankcase. Many years ago, they simply vented the pressure into the atmosphere with a road-draft tube and breather cap. Today we use a positive crankcase ventilation or PCV system to handle this and other task.

How the PCV system works

The PCV system is relatively simple. An inlet hose connects to a filtered air source.  This is used to supply clean air that is drawn through the engine. Most of the time this air is supplied through the engine air filter.  On a few designs there is a separate inlet filter that cleans the incoming air for the PCV system only.  This filtered air flows through the engine, picking up fumes and vapors.  The air exits through another hose, connected to manifold vacuum.  The flow of air draws fumes from the crankcase and burns them harmlessly in the engine.  This also creates a slight vacuum, relieving any pressure that may build.  Negative pressure helps to prevent oil leaks and oil consumption by the engine.  The PCV valve also helps regulate the amount of air flow, which helps prevent oil being drawn out of the engine

PCV system helps remove moisture from oil if driven far enough

The PCV system helps remove moisture, a major contaminant, from the oil

When an engine is run, it generates a good deal of heat. When the engine cools, condensation forms.  Engine oil additives absorb this moisture and hold it in suspension. In time the moisture content exceeds the capacity of the additives.  When this occurs, moisture attacks the metal parts of the engine causing damage. 

Moisture contamination in the PCV system

An outward sign of moisture contamination is a cloudy or milky film in the PCV valve or hose. Finding water in the PCV valve suggests a need for replacement but is also an indication of other problems. Replacing the PCV valve gets rid of the symptom, but the problem remains and symptoms will soon return. If we only drive the vehicle on short trips, moisture content means we need more frequent oil changes and longer drive cycles. A moisture buildup with normal driving shows other engine problems. Several areas of an engine can allow leakage and oil contamination. Coolant leaking into the oil is very serious. Without immediate correction, engine damage is likely.

Moisture contamination flows through the engine oil filter, as it is a liquid.   This is one reason why oil must be changed.  Short trips make the problem far worse as the engine does not reach full temperature.  Oil should be replaced more often when average driving distance is under ten-miles. As the engine reaches full temperature, after about 20 minutes of driving, the heat of the oil causes the moisture to boil. If the vehicle is driven far enough, the PCV system will scavenge much of this moisture from the oil, in the form of steam. This is one reason vehicles can go further between oil changes when the average trip is very long.  With short trips, this does not occur, requiring more frequent oil changes.  The type of driving determines oil change needs, and is a better guide than just the number of miles driven. 

If the PCV system fails, severe sludge buildup and oil leaks can occur

plugged pcv valve allowed moisture buildup resulting in sludge

A plugged PCV valve causes many other engine problems. Pressure begins to build and gaskets and oil seals may fail. When an engine suffers multiple oil leaks, we should always inspect the PCV system. 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.

Symptoms of a bad PCV valve

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.

How a PCV valve works

Most engines employ a PCV valve at the point where fumes are drawn out of the engine. The PCV valve serves several functions. At an idle, engine vacuum is very high, around 16 to 20 inches. This high vacuum would tend to draw oil as well as fumes from the engine. The PCV valve acts as a buffer against oil being drawn out.  It also regulates the amount of vacuum applied to the engine, based on engine load and speed.

Operation of pcv valve under different conditions

At an idle, engine speed is low, around 600 RPM.  A relatively small amount of fuel and air travel through the intake at idle speed. If the PCV valve did not regulate air flow, the engine would act like it had a vacuum leak. Too much air flowing into the intake causes the engine to lean out [too much air in relation to the fuel] and misfire.  At an idle, the PCV valve restricts air flow, to reduce this problem.  At high manifold vacuum [idle], a spring loaded valve is drawn up and partially restricts flow to the crankcase.  The first drawing above illustrates the PCV valve position at idle.

On acceleration far more fuel and air move through the engine and intake manifold vacuum is much lower. Air introduced by the PCV valve has much less influence on the fuel-air mixture.  Low intake manifold vacuum allows the valve to move to a more central position.  In this position the system draws more fumes from the crankcase.  The additional flow is very beneficial, without affecting engine performance.  The center illustration above, shows the PCV valve in acceleration position.

Any pressure in the intake causes flow in the opposite direction. This could occur during an engine backfire or if the engine is turbo-charged.  The PCV valve can act as a check-valve in these situations.  By closing the PCV valve, any positive pressure or fuel vapor is prevented from entering the crankcase. Even a very small amount of positive pressure can force oil passed gaskets and seals and blow gaskets out of place.  Failure of the valve to seal positive pressure may damage the engine.

PCV valve grommets and hoses

Many problems in the PCV system originate with the hoses and mounting components, rather than the valve itself. A PCV valve attaches to the engine in many ways, depending on the design. Manufacturers often use rubber grommets, inserted into a hole in the valve cover. The pliable rubber grommet seals the valve to the cover and holds it in place. On other designs, the valve may screw in or twist-in and seal with on O-ring. Remembering they always seal the valve to the engine is important. Any leak will cause problems, so always inspect closely.

Replacing the PCV grommet with the valve prevents problems

Rubber grommets and O-ring seals get hard over time and cause problems. Grommets sometimes split, creating an oil leak and allowing dirt into the engine. Replacing the grommet or O-ring with the valve prevents many problems. Grommets come in a variety of designs, depending on the engine design. Original equipment grommets work and fit best. If the PCV valve mounts with a grommet, purchase a new one with the valve.

PCV inlet and outlet hoses are also prone to deterioration. Check all hoses in the system when replacing the valve. Hose may become oil soaked and swell, preventing them from sealing. Many hoses get hard with age and crack. A leaking PCV inlet or outlet hose can cause a check engine light or allow debris into the engine.

They design the hoses in the PCV system for vacuum and to be oil resistant. Vacuum hose has a stiff sidewall to resist collapsing. These are very different from fuel hose or heater hose, which they design to hold pressure. Always replace PCV hoses with the original equipment molded hose, from the vehicle manufacturer. Substituting other hose types very often leads to problems and may cause the system to fail, creating oil leaks and allow a sludge buildup.

Collapsed hose blocking PCV flow

Even original equipment hoses sometimes give problems. This is common on Ford and some Mazda vehicles. The hose chosen is not adequate for the task and after miles of use collapses. When this occurs, flow in the system stops and the hole in the hose may create a vacuum leak. Inspect all hoses in the PCV system and replace any that appear soft, swollen or collapsed.

Failure and testing of the PCV valve

As the PCV valve ages several things may happen. Crud and sludge can cause it to stick in the open position. This will produce a vacuum leak and might result in a misfire at idle. Too much air flow causes the engine to lean out, possibly setting a check engine light. Excess flow could also draw oil from the engine, causing oil consumption.

The rattle test give an indication but is not conclusive

Because PCV valves fail in different ways, no test will show all possible problems. For instance, the old test of shaking the valve and listening for a rattle, is only partially helpful. No rattle may show a stuck valve, on many designs, but the valve could rattle freely and still be bad.

Fresh oil in the PCV hose suggest a problem

Another definitive test is to remove the vacuum hose and look for fresh oil. A PCV vacuum hose, with oil dripping or a wet valve usually suggest too much flow, which causes oil consumption. Checking the PCV vacuum hose is a wise precaution, on any engine that consumes oil.

digital manometer can detect a plugged pcv valve 

The PCV valve flows at different rates, under various conditions. For instance, with a high engine vacuum, it should hardly flow at all. Excess flow at an idle will interfere with smooth running. With a lower intake vacuum, flow through the PCV valve increases. A quality auto repair shop will have a tool called a manometer. The manometer measures the very small negative pressure in related to flow. Testing is done at the engine idle speed, under acceleration and under a positive intake condition.

A PCV valve can also stick in a closed position. This allows crankcase pressure and blow-by to build pressure and can damage gaskets and seals. Technicians also test back pressure with a manometer. Positive pressure in the crankcase is a sign of a problem. When an engine begins to develop oil leaks, especially at multiple locations, the PCV system should always be considered.

Extensive testing may be a moot point, as the cost of a replacement valve is normally very low. Cleaning an old valve is much the same. It is rarely effective and replacement of any suspected PCV valve is often far more practical.

PCV valve design variations

For many years the PCV valve remained relatively unchanged. Today a multitude of designs and sizes exist, but most operate in a similar manner. A few manufacturers add heating elements to their PCV valves.  It is thought cold temperature could cause a non-heated valve to freeze and stick, because of moisture drawn through the system.  By heating the valve, freezing is prevented.

heated pcv valve types

Ford uses two designs for heated valves as well as conventional non-heated valves on their engines. One heated design flows engine coolant through tubes to keep the valve warm. Another design is electrically operated. A heating coil inside the valve is used to keep the PCV valve from freezing.

The drawback with heated PCV valves is costs. Heated PCV valves costs many times more than non-heated valves. Most manufactures simply rely on engine heat and the heat of the crankcase vapors to get the job done.

Replacing a PCV valve

Replacing the PCV valve is normally very easy, once the location is found. Most simply push into a rubber grommet. Remove the exit hose and a slight twist breaks them free.  A light pull removes the valve so they can be replaced. Some Ford valves use a quarter-turn system. These are rotated a quarter turn, counter-clockwise before pulling out. A few other designs are actually threaded in and must be unscrewed to remove.

Ford Explorer with 4.0L engine, PCV valve location 

Some PCV valves are also very difficult to access. The 4.0L Ford explorer, in the picture above, has the valve in the rear of the driver’s side valve cover. This can be difficult to find if we do not know the location. To access the valve on a 2.3L Ford Escape we remove the intake manifold. Late model Toyota four-cylinder engines may also place the PCV valve under the intake manifold. With such designs, replacing the valve when we remove the intake for any reason is wise.

Not all engines today use the PCV valve

To lower costs, some manufacturers substitute a restrictor for the PCV valve.  This uses an orifice and a small reservoir to perform some of the functions previously handled by the PCV valve.  A small hole allows enough vacuum to draw fumes from the engine, but not enough to cause a rough idle.  The orifice may become clogged in time and need replacement. Hoses on such a system are also prone to deterioration and have to be replaced when they fail.

A PCV valve often last around 80,000 miles or more and is usually replaced at the first general ignition tune-up. Some can fail much earlier.  Short [under ten miles] trips in the vehicle, will cause the valve to fail sooner.  Under extreme conditions 30,000 mile replacement may be needed. Because of the low cost and ease of replacement on most vehicles, changing the PCV valve is a wise precaution.  If your engine is approaching these mileages or has developed an oil leak, have the PCV system checked. It could save a lot of money.

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