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The PCV valve (Positive CrankcaseVentilation valve)  works as part of the emission control system in a vehicle and functions to control the flow of the fumes in the crankcase and recycle gases produced by combustion in the internal combustion engine. 

The PCV valve can be found between the intake manifold and the crankcase air outlet, often positioned directly at the intake valve cover. 

It is common for an internal combustion engine to use a four-stroke action.  The intake stroke, which is the first stroke of the pistons, draws an equal amount of air from the inlet manifold into each cylinder, delivering the air via the air intake valve. For the next three strokes of the engine the air intake valves are shut while compression, combustion and exhaust processes are occurring. Then the intake valves open again and once more the air is drawn through into the cylinders from the inlet manifold. After the explosion of ignition and the burning of the air and fuel mix in the cylinders, the air and gas remaining is sent back out of the cylinders through an exhaust valve. Some of this gas escapes into the crankcase, known as blow-by, and it is this gas that is sent via the PCV valve to the intake manifold where it is pumped back into the cylinders of the engine where it goes through the  process of combustion again. This process only occurs when the car engine is either at idle or the car is driving at low speed. This is because in these conditions the air pressure in the intake manifold is lower than the air pressure in the crankcase which creates a vacuum which draws the gases through the PCV valve and back into the intake manifold. When the engine returns to top speeds, the PCV valve closes to stop the gases returning back to the intake.

The PCV valve can get clogged up over time which will render it ineffective. Restrictions in the PCV valve can cause fatal damage to the engine. The PCV valve should be kept clean and free of blockages in order for it to function. Oil leaks are one sign that there could be a problem with the PCV valve. It is advisable to seek advice from your mechanic to establish the exact fault so that the correct replacement part/s can be purchased. The PCV valve is designed to be compatible with the make and model your vehicle, providing the perfect replacement part.