Exhaust gas recirculation, or E-G-R systems have been used on automobiles and other vehicles since the 1970's. The main purpose of the system is to control temperatures in your car's combustion chambers, which may help reduce the amount of oxides of nitrogen in exhaust. When nitrous oxides mix with hydrocarbons in the presence of sunlight, smog is what usually results. The E-G-R system recirculates some of the exhaust gases back through the intake manifold to the cylinders and lowers combustion temperature. Your car may have one of two types of E-G-R valve. One operates by using a vacuum, and the other uses pressure to function. Both allow exhaust gas to flow when the temperature gets too high. For more information on the exhaust gar recirculation system in your car, contact a local auto mechanic or auto parts store.