Exhaust Gas Recirculation: Description and Operation

The EGR system returns a small metered amount of engine exhaust gases to the intake manifold where it is mixed with incoming air and fuel. This air/fuel/exhaust mixture lowers the combustion temperature, which reduces the formation of oxides of nitrogen (NOx).

Exhaust gas recirculation is controlled by an EGR control valve. The EGR control valve is installed "in line" in a passage between the exhaust manifold and the intake manifold. Earlier models use external tubes to route exhaust gasses from the exhaust manifold, through the EGR valve, and into the intake manifold, while on later model engines, the gasses are routed through internal passages in the cylinder head. The EGR valve is controlled by a ported vacuum signal, taken from the carburetor. This vacuum signal is modified depending on the vehicle, to control the amount and timing of exhaust gas recirculation.

Fig.11a - Exhaust Gas Recirculation (EGR) System:

The EGR valve (1), Fig. 11a, is controlled by intake manifold vacuum. Exhaust gas recirculation is only possible when the solenoid (2), Fig. 11a, is energized. This can be accomplished only when the following conditions are met: when fifth gear is not engaged (switch (3), Fig. 11a, is open in fifth gear); coolant temperature is above 113'F (coolant temperature switch (4), Fig. 11a, is closed); the pressure differential switch (5), Fig. 11a, is electrically fed (chamber A vacuum greater than chamber B vacuum, Fig. 11a).