It is normal for a turbocharged or supercharged car to build up pressure in its crankcase due to “blow-by”.  In order to relieve this pressure, auto manufactures have designed a PCV, or “Positive Crankcase Ventilation” system.  This routs this hot vapor of air and oil back into the intake track so that it can be burned and sent through the car’s catalytic converter.  However, this oil vapor can build up, and cause carbon deposits in the engine, and reduce the efficiency of the turbo and intercooler.