Exhaust Gas Recirculation: Description and Operation

Exhaust gas recirculation is used to control part-throttle exhaust emissions. During recirculation, a small amount of exhaust gas enters the intake manifold and this allows a reduction in the useful volume of the fuel/air mixture. This smaller volume means lower combustion temperatures, which reduce the formation of oxides of nitrogen.
A temperature valve and a vacuum amplifier control the EGR valve. The amplifier works like a vacuum relay which uses engine vacuum to operate the EGR valve, but is controlled by weak venturi vacuum produced by a special restriction near the throttle plate.
At idle there is not enough control vacuum to operate the amplifier. At part throttle, control vacuum trips the amplifier and allows engine vacuum to operate the EGR valve. At full throttle the EGR valve is closed because engine vacuum has dropped off even though control vacuum continues to increase with engine speed. If a malfunction occurs in the EGR system an indicator light, located on the instrument panel, will illuminate.