420 & 560

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 is vented to the atmosphere using a connection in the throttle valve housing, thermovalve and vacuum control valve. If throttle valve is partially opened, vacuum is routed to the EGR valve the same way and the EGR valve opens.

At high engine loads (not full load), the vacuum control valve keeps the EGR valve open even though it would close by its relationship between spring force and low vacuum. The bore is closed by the diaphragm in the vacuum control valve and sufficient vacuum is maintained to keep the EGR valve open.

Under full load conditions, the vacuum is reduced to the point where the vacuum control valve vents the EGR valve. The connection of the vacuum control valve serves to vent the EGR valve quickly, thereby closing it during transition from partial load operation to idle speed.