Most 1973 and later model vehicles are equipped with an exhaust gas recirculation system to recirculate proper amounts of exhaust gases into the air/fuel mixture in the intake manifold. This recirculation into the manifold reduces combustion temperatures, thus reducing oxides of nitrogen (NOx). There are several types of EGR systems: a back pressure system which operates in conjunction with exhaust back pressure, a conventional system which operates on carburetor venturi vacuum, an electronically controlled system which operates from the engine computerized emissions control system, or a floor jet system which operates through jets in the floor of the intake manifold.