When the pistons move up and down during combustion crankcase pressure is developed underneath each piston. The pistons are operating in a partially closed system that van generate tremendous unwanted crankcase pressure. The positive crankcase pressure relief system is designed to relieve the crankcase pressure and recycle the gases created by the interaction of the piston rings within the combustion chamber.

The positive crankcase pressure system allows controlled and directed crankcases to be reused and burned in the combustion chamber. Fresh air is drawn in to the engine through the breather hoses near the air intake. A PCV valve regulates the quantity of recycled crankcase by opening more during high crankcase pressure conditions and less during low crankcase pressure conditions. The crankcase gases are acidic because combustion contains toxic gases like sulfur dioxide and carbonic acid. The hoses of the PCV system are designed to withstand the harshness of the toxic fumes. The PCV design was the very first smog control. Prior to 1968 many cars had open PCV systems called "road draft" which means the PCV gases were open tubes that allowed crankcase gases to vent into the atmosphere and onto the road. Modern vehicle design have more complex PCV systems that filter PCV gases before they are reintroduced into the combustion chamber.

The oil is eventually contaminated because of exposure to crankcase gases and PCV. Engine oil with high mileage is more acidic and corrosive. Old acid oil that is corrosive is being recycled to the combustion chamber through the PCV system. Therefore it is important to replace the engine oil at the manufacturers recommended service intervals. Clean engine oil will protect the engines moving parts and keep the combustion chamber clean. Combustion chambers that use excessively dirty oil become carbon fouled. Carbon fouled engines act like charcoal from a barbecue. The carbon can pre ignite the fuel because it collects on the top of the pistons and on the valve seats.