During the internal combustion process, some of vapor will somehow pass through the very thin space between the cylinder and piston. This vapor is called engine blow-by gas, in general. This blow-by gas is then recirculated back into the air intake to the engine room for combustion with the help of PCV, positive crankcase ventilation system.

If the PCV valve is either stuck open or closed, the engine could mechanically damanaged permanently. Although PCV is a small device and does not cost you a lot, the role of the PCV is so crucial. Especially for GDI Turbo charged engines, PCV valve must be inspected regularly.