The positive crankcase ventilation system was one of the first emission control devices to be used on engines.
Before the introduction of the PCV, all engines had a “breather tube” that remove crankcase vapors and gases from the engine into the atmosphere, but today, engines use some variation of the positive crankcase ventilation system to stop crankcase vapors and gases, produced within the engine, from getting into the atmosphere.

It has seriously reduced blow-by emissions from the engine. About 20% of the total hydrocarbon (HC) emissions produced by a vehicle are blow-by emissions from gases that get past the piston rings and enter the crankcase. The higher the mileage on the engine and the greater the wear on the piston rings and cylinders, the greater the blow-by into the crankcase.
Positive crankcase ventilation involves recycling gases (blow-by gases) through the PCV valve to the intake manifold, where they’re pumped back into the cylinders for another shot at combustion.
It isn’t always desirable to have these gases in the cylinders because they tend to be mostly air and can make the gas-air mixture in the cylinders a little too lean for effective combustion. The blow-by gases only recycles when the car is traveling at slow speeds or idling. At idle the air pressure in the intake manifold is lower than the air pressure in the crankcase, it’s this lower pressure (which sometimes approaches pure vacuum) that sucks the blow-by gases through the PCV valve and back into the intake. When the engine speeds up, the air pressure in the intake manifold increases and the suction slows down, reducing the amount of blow-by gas recycled to the cylinders.
This is good, because the blow-by gases aren’t needed when the engine speeds
up. In fact, when the car is up to speed, the pressure in the intake manifold can actually become higher than the pressure in the crankcase, potentially forcing the blow-by gases back into the crankcase.
Since the whole point of positive crankcase ventilation is to keep these gases out of the crankcase, the PCV valve is designed to close off when this happens and block the back-flow of gases.
The PCV system is designed to work in two ways;
1. The Heated system
2. Non Heated system.
The Heated PCV system uses either a water heated valve or an electrically controlled heated valve or an electrically controlled heater tube. This system is mainly controlled by the PCM/ECU or by Thermal reaction while the Non heated system is mainly controlled by vacuum.
Operation of the PCV vacuum: Air from the system enters the air cleaner duct. The air then goes through the air filter, through a tube, and through the closed oil filler cap. The intake manifold vacuum then draws the crankcase vapors and gases back to the PCV valve. From the PCV valve, the vapors and gases are drawn into the intake of the engine to be burned by combustion. If too many vapors and gases get into the intake manifold, it may upset the air-fuel ratio.
The PCV valve controls the amount of vapors and gases going back into the intake manifold. A typical PCV valve is shown below.
In operation, two forces are working against each other. Spring pressure inside the PCV valve works against the intake manifold vacuum. When the engine is stopped, no intake manifold vacuum exists. At this point, the PCV valve is moved down by the spring inside.
At deceleration or idle, intake manifold vacuum is very high. The vacuum lifts the PCV tapered valve upward against the spring pressure which reduces the size of the metered opening. At this point very little crankcase vapors or gases enter the intake manifold.
At normal loads and speeds, the vacuum in the intake manifold drops and allows the inside spring to push the plunger down which increases the metered opening on the top of the valve. At this point the amount of crankcase ventilation vapors and gases going into the intake manifold increases.
At acceleration or heavy loads, the intake manifold vacuum is very low and inside spring now pushes the tapered metering valve further down, allowing more crankcase vapors and gases to enter the intake manifold. This implies that, when the engine is at low speeds, only a small amount of crankcase vapors and gases are sent into the intake manifold. As the engine increases in speed and load, more and more crankcase vapors and gases are allowed to enter the intake manifold. This operation depends on the design and type of PVC system applied by the engine manufacturers.
The major issue that affects PCV systems is a clogged up PCV valve. This occurs by an accumulation of fuel and oil varnish deposits and/or sludge inside the valve which can restrict or even block the flow of vapors through the valve. A restricted or clogged PCV valve cannot pull moisture and blow-by vapors out of the crankcase.
This can cause the formation of engine-damaging sludge, and a backup of pressure that may force oil to leak past gaskets and seals.
The loss of airflow through the valve or a shut pintle inside the PCV valve can also cause the air/fuel mixture to run richer than normal, increasing fuel consumption and emissions. 
Likewise, If the pintle inside the PCV valve sticks open, or the spring breaks, or the hose that connects the valve to the throttle body, carburetor or intake manifold pulls loose, cracks, or leaks, 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). 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.
In recent times most vehicles are designed with computer engine controls/management systems, the engine management system monitors changes in the air/fuel mixture and compensate by increasing or decreasing short term and long term fuel trim (STFT and LTFT).
Compensation more that + or – 15 points, will typically set a lean or rich DTC and turn on the MIL.
Installation of the wrong PCV valve for the application can also cause an issue bearing in mind that 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. So be very sure of the specifications for the part you are buying.
How to confirm a functional PCV valve: The best way to test the PCV valve is with the use of a FLOW TEST, results are best if we have confirmed there is no broken or leaking hose and that the system is not clogged and also confirming that the PINTLE is not stuck open or close or has a rattling sound when you shake it.
You can also check by holding your finger over the end of the valve while the engine is idling.
How to confirm functional PVC system: Being able to control the volume of air pulled form the crankcase by the PVC system is very important, due to the fact that a specified amount of airflow is required to remove blow-by vapors and moisture. When you have it in excess or reduced a problem must show up.
1. If we Pinch or block off the vacuum hose to the PCV valve while the is engine idling at operating temperature. The engine idle rpm 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.
2. Measure the amount of vacuum in the crankcase with a flow meter, at normal engine 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 pulls about 1 to 3 inches of vacuum in the crankcase at idle. If you notice 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 is not pulling enough air through the crankcase to get rid of the blow-by vapors.
We will continue with a case study in the next article.
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