The exhaust gas recirculation (EGR) system allows burned exhaust gases to enter the engine intake manifold to help reduce NOx emissions. When exhaust gases are added to the air-fuel mixture, they decrease peak combustion temperatures. For this reason, an exhaust gas recirculation system lowers the amount of NOx in the engine exhaust.
A basic EGR system is simple. It consists of a vacuum Operated EGR valve and a vacuum line from the carburettor. This is pictured in Fig. 31-14. The EGR valve usually bolts to the engine intake manifold or a carburettor plate. Exhaust gases are UM routed through the cylinder head and intake manifold to the EGR valve.
The EGR valve consists of a vacuum diaphragm, spring, plunger, exhaust gas valve, and a diaphragm housing. It is designed to control exhaust flow into the intake manifold. See Fig. 31-15.