Several gases and vapors are formed during the process of combustion, and some of them can be very corrosive. If these gases weren't vented from your car's engine, they could cause damage such as rust, corrosion, or the formation of sludge. Prior to the seventies, gases from the engine were simply released into the atmosphere through a tube. When people began to realize the serious threat pollution was becoming, car manufacturers developed a system of returning the gases back into the combustion chambers for burning. The positive crankcase ventilation system consists of a valve that controls the amount of gases re-circulated in the engine according to engine speed, and a hose connected between the crankcase and the intake manifold. It's important to get your car's p-c-v valve replaced periodically as it may become clogged with sludge and soot. Contact a local auto mechanic for more information on the positive crankcase ventilation system.