In the process of combustion, several gases and vapors are formed, and some of them could potentially cause damage to your engine if they're not vented. These gases could lead to corrosion, rust, or the formation of sludge. In the past, these gases were released into the atmosphere through a tube. When pollution began to be a noticeable problem in the seventies, a system called positive crankcase ventilation was developed and installed on cars. This system returns the gases to the combustion chambers to be burned. The positive crankcase ventilation system uses a p-c-v valve that regulates the amount of gases re-circulated according to engine speed. Your car's p-c-v valve should be changed periodically as it may become clogged with sludge or soot and work less efficiently. Contact a qualified local mechanic or auto parts store for more information on positive crankcase ventilation.