Exhaust Gas Recirculation: Description and Operation

Fig. 2, Air injection, EGR, and Fuel Evaporation system.:

This EGR system Fig. 2, is vacuum operated, and electronically controlled. The system consists of an electronic control unit, vacuum control valve, switchover valves, microswitches, coolant temperature switch, vacuum control flap, throttle valve housing, and a TDC transmitter.
The EGR system operates under the following conditions:
1. Engine temperature above 122°F (50°C).
2. Under all engine conditions except idle and full load.
With the throttle valve closed, the EGR valve actuating circuit is vented to the atmosphere through a connection in the throttle valve housing, thermo-valve, and vacuum control valve. If the throttle is partially opened, the circuit is closed and vacuum is routed to the EGR valve causing it to open.
At less than full throttle, but with a high engine load, the vacuum control valve holds the EGR valve open even though the low vacuum would normally allow the valve to close.
Under full load conditions, the vacuum is reduced to the point where the vacuum control valve vents the vacuum circuit to the atmosphere. The vacuum control valve vents quickly, closing the EGR valve during transition from partial load operation to idle speed.