GM Full-Size Trucks 1970-1979 Repair Guide

Exhaust Gas Recirculation (EGR)


See Figure 1

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Fig. Fig. 1: Common diesel engine EGR valve location

To lower the formation of nitrogen oxides (NOx) in the exhaust, it is necessary to reduce combustion temperatures. This is done in the diesel, as in the gasoline engine, by introducing exhaust gases into the cylinders through the EGR valve.


Vacuum Regulator Valve (VRV)

The Vacuum Regulator Valve 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 is reduced as the throttle is opened. At closed throttle, the vacuum is 15 in.Hg; at half throttle, 6 in.Hg; at wide open throttle there is zero vacuum.

Exhaust Gas Recirculation (EGR) Valve

Apply vacuum to vacuum port. The valve should be fully open at 10.5 in.Hg and closed below 6 in.Hg.

Response Vacuum Reducer (RVR)

Connect a vacuum gauge to the port marked "To EGR valve or T.C.C. solenoid." Connect a hand operated vacuum pump to the VRV port. Draw a 50.66 kPa (15 in.Hg) vacuum on the pump and the reading on the vacuum gauge should be lower than the vacuum pump reading as follows:

0.75 in.Hg Except High Altitude
2.50 in.Hg High Altitude

Torque Converter Clutch Operated Solenoid

When the torque converter clutch is engaged, an electrical signal energizes the solenoid allowing ports 1 and 2 to be interconnected. When the solenoid is not energized, port 1 is closed and ports 2 and 3 are interconnected.

Engine Temperature Sensor (ETS)

See Figure 2

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Fig. Fig. 2: View of the Engine Temperature Sensor (ETS)

The engine temperature sensor has two terminals. Twelve volts are applied to one terminal and the wire from the other terminal leads to the fast idle solenoid and Housing Pressure Cold Advance solenoid that is part of the injection pump.

The switch contacts are closed below 125°F. At the calibration point, the contacts are open which turns off the solenoids.