1975-76 450 Models: Notes
The EGR system works in 2 stages. The first stage permits a small amount of exhaust gas recirculation, while second stage permits full exhaust gas recirculation. Both stages are controlled by coolant temperature, vacuum control switch, and throttle position.
First stage EGR takes place during driving operations with coolant temperatures above 104°F (40°C). The thermo/vacuum valve is open and vacuum from throttle valve is available at EGR valve. See Fig 1. The EGR valve opens and exhaust gases are drawn into the intake manifold.
The second stage of the EGR valve operates during periods of acceleration when coolant temperature is above 104°F (40°C). During acceleration, vacuum drops and vacuum control switch directs vacuum to second stage of the EGR valve. The EGR valve opens completely. When vacuum rises above 8 in. Hg, the vacuum control switch will close off vacuum to second stage of EGR valve by venting vacuum to the atmosphere. At this point, the EGR valve operates in first stage.