See Figures 1 and 2
The diesel EGR systems work in the same basic manner as gasoline engine EGR systems: exhaust gases are introduced into the combustion chambers to reduce combustion temperatures, and thus lower the formation of nitrogen oxides (NOx).
Vacuum from the vacuum pump is modulated by the Vacuum Regulator Valve (VRV) mounted on the injection pump.
The amount of EGR valve opening is further modulated by a Vacuum Modulator Valve (VMV). The VMV allows for an increase in vacuum to the EGR valve as the throttle is closed, up to the switching point of the VMV. The system also employs an RVR valve.
In 1985, the EGR valve was moved to the rear of the engine and has become electronically controlled.
SERVICE (COMPONENT TESTING)
See Figure 3
The VRV is attached to the side of the injection pump and regulates vacuum in proportion to throttle angle. Vacuum from the vacuum pump is supplied to port A and vacuum at port B (see illustration) is reduced as the throttle is opened. At closed throttle the vacuum should be 15 in. Hg (51 kPa), and at half throttle 6 in. Hg (20 kPa); at wide open throttle there should be zero vacuum.Exhaust Gas Recirculation Valve
Apply vacuum to the vacuum port. The valve should be fully open at 12 in. Hg (41 kPa) and closed below 6 in. Hg (20 kPa).Response Vacuum Reducer (RVR)
Connect a vacuum gauge to the port marked To EGR valve or TCC solenoid. Connect a hand operated vacuum pump to the VRV port. Draw 15 in. Hg (51 kPa) of vacuum on the pump and the reading on the vacuum gauge should be lower than the vacuum pump reading by 0.75 in. Hg (2.5 kPa).Exhaust Pressure Regulator Valve
Apply vacuum to the vacuum port of the valve. The valve should be fully closed at 12 in. Hg (41 kPa) and open below 6 in. Hg (20 kPa).Vacuum Modulator Valve (VMV)
To test the VMV, block the drive wheels, and apply the parking brake. With the shift lever in Park, start the engine and run at a slow idle. Connect a vacuum gauge to the hose that connects to the port marked MAN . There should be at least 14 in. Hg (47 kPa) of vacuum. If not, check the vacuum pump, VRV, RVR, solenoid, and all connecting hoses. Reconnect the hose to the MAN port. Connect a vacuum gauge to the DIST port on the VMV. The vacuum reading should be 12 in. Hg (40 kPa) except on High Altitude cars, which should be 9 in. Hg (30 kPa).