PCV Valves (Positive Crankcase Ventilation) work by allowing the blow-by gasses (air-fuel mixture that escapes the combustion chamber and enters the crankcase) to be recirculated into the intake manifold and ultimately the combustion chamber. As the pressure in the intake manifold greatly increases (going from a vacuum at idle to high pressure when the engine is at high RPM), the PCV Valve prevents air from being pushed back into the crankcase. It is essentially a one way valve - allowing air out but not back in - hence the name. PCV Valves were the first real emissions system to come into play on a vehicle, as this escaped air-fuel mixutre used to be vented into the atmosphere.
A PCV Valve is a relatively inexpensive part, and labor is usually not that bad either. They can get clogged easily from all the particles held in the air that is passed through them. Many mechanics recommend having them replaced once per year.
There are service manuals which provide detailed step by step instructions on how to replace a PCV valve for the do-it yourself mechanics. Otherwise it should be a fairly cheap repair for any service shop!
We stock and sell a wide variety of quality PCV Valves from these manufacturers: