The PCV valve is one half of the crankcase ventilation system ? the other half is the breather filter. The breather flows clean air into the crankcase through a hose on your air filter and the PCV valve lets dirty air and crankcase gases out. These dirty gases and air get sent through the engine to be re-burned for clean emissions. An old PCV valve can cause rough engine idle, blown oil seals, fouled air filter and fuel system, fumes in the passenger compartment, excessive engine part wear, and shorter oil life. Although the PCV is not a pollution control valve, a worn out PCV valve can contribute to air pollution. So, it's important to change your PCV valve for the safety of your vehicle.