Positive Crankcase Understanding
History of the PCV – Positive Crankcase Ventilation valve
Before the 1960s car engines were vented to the atmosphere. That is, toxic vapors that were created by exhaust gases leaking past the rings (called “blow-by”) and into the crankcase were simply allowed to flow out of the engine. PVC was, usually, accomplished by a metal tube that routed from the top of the engine down underneath. The air flowing under the car helped to draw vapors out. As engines aged, those vapors contained more and more soot and other contaminates that contributed to smog and overall pollution.
GM researchers identified engine blow-by gas as a major source of hydrocarbon emissions and developed the Positive Crankcase Ventilation valve, commonly known as the PCV valve, to cap the leak. Made standard on all GM cars sold in the U.S. beginning in 1963, it was the industry’s first vehicle emissions control device. Before 1963 PCV was only used in California. There is a variety of PCV systems used on various makes and models of cars produced since 1963, but all function essentially the same.
PCV control valve action
Vapor is then carried with the fuel/air mixture into the combustion chambers where it is burned. Since the manifold vacuum is constantly changing, some sort of control must be in the system. This control device is the Flow Control Valve, commonly referred to as the PCV Valve.
PCV systems can be described as either open or closed. The two systems are quite similar. However, the closed system in use since 1968 is more effective at air pollution control. The systems differ in the manner in which fresh air enters the crankcase and excessive vapor is expelled.
Positive Crank Case Understanding
Open PCV Systems
The open system draws fresh air through a vented oil filler cap; usually, chrome-plated in restored cars. This works fine as long as the vapor volume is minimal and when the engine is running. However, when the crankcase vapor becomes excessive – or when the engine is shut off – it is forced back through the vented oil filler cap and into the open atmosphere. The open PCV system, though successful at removing contaminated vapors from the crankcase, is not completely effective as a pollution control device.
Closed PCV Systems
The closed PCV system draws fresh air from the air filter housing. The oil filler cap in this system is NOT vented. Consequently, excess vapor will be carried back to the air filter housing and from there into the intake manifold. The closed system prevents vapor, whether normal or excessive, from reaching the open atmosphere. The closed system is very effective as an air pollution control device.
The PCV Valve – More Complicated Than You Think
The purpose of the PCV valve is to meter the flow of the vapor from the crankcase to the intake manifold. This is necessary to provide proper ventilation for the crankcase while not upsetting the fuel/air mixture for combustion.
Blow-by gases and vapor should be removed at about the same rate they enter the crankcase. Since blow-by is minimal at idle and increases during high-speed operation, the PCV valve must control the flow of vapor accordingly. The PCV valve is designed to compensate for the engine ventilation needs at varying engine speeds. It is operated by manifold vacuum, which increases or decreases as engine speeds and loads change.
For example, at low or idle engine speeds manifold vacuum is high. This pulls the plunger to the extreme-forward position or manifold end of the valve. Due to the shape of the plunger, vapor flow is reduced to a minimum. The low rate of the flow is adequate for ventilation purposes and will not upset the fuel/air mixture ratio.
At high speeds, the manifold vacuum is decreased. The plunger is only drawn to a point about midway in the housing. This allows a maximum flow of vapor. Since the engine needs more fuel/air mixture at high speeds, the introduction of more vapor does not significantly affect performance. In the event of a backfire, pressure from the intake manifold forces the plunger to the closed or engine-off position. This prevents the backfire flame from reaching the crankcase and exploding the combustible vapor.
Okay? Now What If It Isn’t Working Properly?
A neglected PCV system will soon fail to function, and the result can be expensive as well as troublesome for the car owner. If the crankcase is not adequately ventilated, the motor oil will become contaminated and heavy sludge accumulations will begin to form. Internal parts, not protected by the motor oil, will begin to rust and/or corrode due to the water and acids that will become trapped within the crankcase.
If the PCV system is not functioning properly, the flow of crankcase vapor into the intake manifold will not be properly metered. This, in turn, will upset the fuel/air mixture for combustion and can cause rough idling or even stalling of the engine. Furthermore, intake and exhaust valves, in addition to sparking plugs, may eventually be burned and rendered useless, prematurely affecting performance and requiring expensive repairs. To assure trouble-free performance of the PCV system and, in turn, the engine and vehicle, routine maintenance of the PCV system is recommended and required.
Millions of owners think that if a PCV valve rattles when shaken that it is okay. Wrong! Just because it rattles doesn’t mean its calibrated spring is metering correctly. Cleaning the PCV doesn’t accomplish anything either. A PCV valve should never be cleaned and placed back into service. Cleaning the PCV valve will result in a clean PCV valve; not a new PCV valve.
Some contaminants will remain in the PCV valve that can never be flushed out. Additionally, there is an amount of wear that will be experienced by the spring that cleaning cannot replace.
What are the Symptoms of a Bad PCV VALVE?
The positive crankcase ventilation or PCV valve is inexpensive and the part most consumers overlook. It is also one of the possible causes of expensive oil leaks and sludge buildup inside the engine.
All automotive engines are lubricated with oil, and when oil is churned by moving parts, the pressure is produced by combustion. Piston rings and valve guides also leak slightly producing pressure, called Blow-by, in the crankcase. Many years ago, the engines would simply vent the pressure into the atmosphere with a road-draft tube and breather cap. Today we use positive crankcase ventilation or PCV system to handle this, and also to help lower the harmful emissions engines produce.
The most common problem that afflicts the PCV systems is a plugged up PCV valve or hose. 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 plugged PCV valve cannot pull moisture and blow-by vapors out of the crankcase. The valve can cause engine-damaging sludge to form and the 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.
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. The PCV 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 “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.
On late-model vehicles with computerized 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. Quality OEM brand replacement PCV valves are calibrated to the specs the manufacturers designed them to operate in, which provide long-lasting, trouble-free performance.
Note: On many 2002 and newer vehicles with OBD II. the OBD II system monitors the PCV system and checks the flow rate once during each drive cycle. But on older OBD II and OBD I systems, the PCV system is NOT monitored. So the problem with the PCV system on a pre-2002 vehicle probably won’t turn on the MIL
(malfunction indicator lamp) or set a diagnostic trouble code (DTC).
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 a 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 airflow, which helps prevent oil from being drawn out of the engine.
Note: PCV system helps remove moisture from oil if driven far enough
The PCV Valve system helps remove moisture, a major contaminant, from the oil
When the engine is running, it generates a great deal of heat and as the engine cools, condensation forms inside the engine. Engine oil additives help absorb this moisture and keep it in suspension. If the moisture content exceeds the capacity of the additives, it will start to attack the metal parts of the engine causing internal engine damage. Keeping up with your regular Oil Changes will help reduce moisture.
Moisture contamination in the PCV system
A sign of engine moisture contamination will show a cloudy or milky film in the PCV valve or hose. If you find water in the PCV valve system, and its hoses, suggests a need for replacement but is also an indication of other problems. Replacing the PCV valve will help get rid of some problems, but the main problem remains, and symptoms will soon return. If we only drive a 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, such as Head Gasket, or Cracks, or Intake Manifold Gasket problems Several areas of the engine can allow leakage and oil contamination. Engine Seals and Valve Cover gaskets are the most common areas to leak first. Coolant leaking into the oil is a very serious problem. Without immediate correction, engine damage is likely to occur.
Note: the milky film seen in the PCV Valve system is due to a chemical reaction from the anti-freeze. If no anti-freeze is present in the cooling system, you will not see the whiteness. You have seen oil spills in the ocean from the news, and the oil is still black in color floating on the ocean top, correct?
The engine’s oil filter helps to remove the contaminants from the oil, which are by-products of combustion and moisture. It’s these things that cause internal engine problems over time if the oil and filter are not changed regularly. This is one reason oil changes are a must. Short trips make the problem far worse as the engine does not reach full temperature. Oil and filter should be replaced more often when the 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 off and out through the suction action of the PCV Valve. If the vehicle is driven far enough, the PCV system will pull 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 PVC 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, You should always inspect the PCV system. Another problem is a lack of airflow to carry vapors from the crankcase. Without airflow, moisture contamination remains, and a sludge buildup is often the result. Operating the engine without adequate ventilation is a leading cause of engine sludge.
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, the engine vacuum is very high, around 16 to 20 inches (Hg). 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 the engine’s load and speed.
Operation of PCV valve under different conditions
At an idle, the 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 does not regulate airflow, 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 a high manifold vacuum (idle), the 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 engine acceleration, more fuel and air move through the engine and intake manifold and the vacuum is much lower. Air introduced by the PCV valve has less of an effect on the fuel-air mixture. A low intake manifold vacuum allows the valve to move to a more central position. In this position, the system draws more combustion vapors from the crankcase. The additional flow is very beneficial, without affecting engine performance. The center illustration above shows the PCV valve in the acceleration mode position.
Any pressure in the intake causes flow in the opposite direction. The action of the PCV valve to the pressure will occur during an engine backfire, engine miss, or if the engine is turbo-charged. The PCV valve can act as a check-valve in these situations. By the PCV valve closing, any positive pressure, the fuel vapor is prevented from entering the crankcase. Even if a very small amount of positive pressure can force oil passed gaskets and seals and cause oil and vacuum leaks. Failure of the valve to seal positive pressure may damage the engine.
PCV valve grommets and hoses
Many problems in the PCV Valve system originate from 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. The PCV valve must be completely sealed for maximum benefit, very important! Any leak will cause problems, so always inspect the positive crankcase system 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 crack and 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 manufacturer (OEM) 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(s) may become oil soaked and swell, preventing them from sealing the engine. 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.
Automotive manufacturers design the hoses in the PCV system for vacuum and to be oil resistant. The 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 the PCV Valve hoses with the original equipment molded hose, from the vehicle manufacturer. Substituting any other hose types very often leads to problems and may cause the positive crankcase ventilation system to fail, creating oil leaks and allow a sludge buildup.
Collapsed PCV hose will block, or restrict the PCV Valve Vacuum 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, it will collapse. When this occurs, flow to the PCV valve system stops and the hole in the supply 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. Gunk and sludge can cause the valve to stick in the open position. Eventually, will produce an engine vacuum leak and might result in a misfire at idle. Too much airflow causes the engine to lean out, possibly setting a check engine light. The 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 the 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. Use Best Practices and change the valve. This little cheap part, if not changed in time, will cost thousands of dollars in engine repairs if not changed at the recommended interval.
Fresh oil in the PCV hose suggests a problem
Another definitive PCV valve test is to remove the vacuum hose and look for fresh oil. A PCV vacuum hose, with oil dripping or a wet valve, usually suggests too much flow, which causes oil consumption. Checking the PCV vacuum hose is a wise precaution, on any engine that consumes oil. Also, if you see blue smoke coming from the tailpipe can mean many reasons, but the first step is to check the PCV valve. Basics First!
A digital manometer can detect a plugged PCV valve
The PCV valve flows at different rates, under various driving conditions. For instance, at a high engine vacuum, the valve should hardly flow at all. An Excess flow at an engine 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 is 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 the closed position, which allows crankcase pressure and blow-by to build up pressure and can damage gaskets and seals. Auto Technicians also test back pressure with a manometer. Positive pressure in the crankcase is a sign of a problem. When the 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 the 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 similarly. 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. Heating keeps the PCV valve from freezing is prevented.
Ford uses two designs for heated PCV 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 of heated PCV valves is cost. Heated PCV valves cost many times more than non-heated valves. Most manufacturers simply rely on the engine, and crankcase vapors, heat to get the job done.
Replacing a PCV valve
Replacing a PCV valve is normally very easy to do, 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 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, and others not. Take, for instance, the 4.0L Ford Explorer, in the picture above, has a valve in the rear of the driver’s side valve cover. While it can be difficult to find the PCV valve, especially if we do not know the location, you can ask an automotive technician here on www.autorepairhelp.us to know where your positive crankcase valve is. 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 vast designs, replacing the valve when we remove the intake manifold for any reason is wise.
Not all engines today use the PCV valve
Some manufacturers lower the cost of material by substituting a restrictor for the PCV valve. This PCV valve 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 lasts 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, a 30,000-mile replacement may be needed. Because of the low cost and easy to replace on most vehicles, changing the PCV valve is a wise decision. If your engine is approaching these mileages or has developed an oil leak, have the PCV system checked as soon as possible. It could save a lot of money in the long run. After all your vehicle is your second best investment.
Modified and Additional information by Master Tech Lee
If you need help replacing your PCV valve please contact us and we will provide you with the replacement interval and help you step by step for free.
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