Gasoline Engines
The EGR system operates under the following conditions: engine temperature above 122°F (50°C), under all engine load conditions except at idle and full load.
With the throttle valve closed, the EGR valve actuating circuit is vented to the atmosphere using a connection in the throttle valve housing, thermovalve and vacuum control valve. If throttle valve is partially opened, the circuit is closed and vacuum is routed to the EGR valve and the valve opens.
At less than full throttle, but with a high engine load, the vacuum control valve holds the EGR valve open even though the reduction in vacuum would normally allow the valve's return spring to close the valve.
Under full load conditions, the vacuum is reduced to the point where the vacuum control valve vents the EGR valve. The vacuum control valve vents the EGR valve quickly, thereby closing it during transition from partial load operation to idle speed.