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. There is also a small leakage passed piston rings, that produces positive pressure in the crankcase. Many years ago, this pressure was simply vented into the atmosphere. Today the PCV system handles the tasks.
The PCV system is relatively simple. An inlet hose allows filtered air to enter the engine. Most of the time this air is drawn through the engine air filter. On a few designs there is a separate inlet filter. The filtered air flows, and exits through another hose, connected to manifold vacuum. Engine vacuum draws fumes from the crankcase and burns them harmlessly in the engine. Relieving this pressure helps to prevent oil leaks and oil consumption.
The PCV system helps keep moisture, a major contaminant, out of the oil
When an engine is run, a good deal of heat is generated. As the engine cools, condensation forms. The engine oil absorbs this moisture and attempts to hold it. After a period of time the moisture becomes too much and begins to attack the metal parts of the engine.
Moisture is not remove by the engine oil filter as it too is a liquid. This is one reason why oil must be changed and why with short trips, it must be changed more often. As the engine reaches full temperature, after about 20 minutes of driving, the heat of the oil causes the moisture to begin boiling. 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, far more than the number of miles driven.
If the PCV system fails, severe sludge buildup and oil leaks can occur
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 and regulates the amount of vacuum applied to the engine.
Because the engine idles at low speed there is a relatively small amount of fuel and air traveling through the intake. If the PCV valve did not regulate flow, the engine would act like it had a vacuum leak. Air flowing into the intake might cause 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. On acceleration there is far more volume of fuel and air moving through the engine. Intake manifold vacuum is also much lower. This allows the valve to move to a more central position, increasing fumes drawn from the crankcase. This produces far more removal of fumes, without affecting engine performance.
Should the engine backfire or if the engine is turbo-charged, the PCV valve can also act as a check-valve. Pressure in the intake would cause flow in the opposite direction. 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.
As the PCV valve ages several things can happen. Crud and sludge can cause it to stick in the open position. This might result in a misfire at idle. Too much air flow acts like a vacuum leak. Excess flow could also draw oil from the engine, causing oil consumption. A quality shop can test this using a tool called a manometer.
The PCV valve can also stick in a closed position. This can allow crankcase pressure and blow-by to damage gaskets and seals. This too can be tested for with a manometer. When an engine begins to develop oil leaks, especially at multiple locations, the PCV system should always be checked.
For many years the PCV valve remained relatively unchanged. There are a multitude of designs and sizes, but most operate similarly. Recently a few manufacturers have begun to add heating elements to their PCV valves. Because of moisture drawn through the system, cold temperature could cause a non-heated valve to freeze and stick.
Ford uses two designs as well as conventional non-heated valves on their engines. One design flows engine coolant through tubes to keep the valve warm. Another design is electrically operated. A coil inside the valve is used to keep the PCV valve heated.
The obvious drawback with heated PCV valves is cost. Heated valves can cost 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 the PCV valve is normally very easy. Most simply push into a rubber grommet. Remove the exit hose and a slight twist breaks them free so they can be pulled out and replaced. Some Ford valves use a quarter-turn system. These are rotated a quarter turn, counter-clockwise to remove. A few other designs are actually threaded in and must be unscrewed to remove.
Not all engines today use the PCV valve. To lower costs, some have substituted a restrictor. This uses an orifice to perform some of the functions previously handled by the valve.
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 as much earlier. Short [under ten miles] trips will cause the valve to fail sooner. Under extreme conditions 30,000 mile replacement may be needed. 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.