GM Full Size Vans 1967-1986 Repair Guide

Exhaust Gas Recirculation (EGR)



The EGR system and valve were introduced in 1973. Its purpose is to control oxides of nitrogen which are formed during the peak combustion temperatures. The end products of combustion are relatively inert gases derived from the exhaust gases which are directed into the EGR valve to help lower peak combustion temperatures.

The EGR valve contains a vacuum diaphragm operated by manifold vacuum. The vacuum signal port is located in the carburetor body and is exposed to engine vacuum in the off/idle and part throttle operation. In 1974, a thermal delay switch was added to delay operation of the valve during engine warmup, when NOx levels are already at a minimum.

On inline sixes, the EGR valve is located on the intake manifold adjacent to the carburetor. On V6 and V8 engines, the valve is located on the right rear side of the intake manifold adjacent to the rocker arm cover.


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Some EGR valves may be tested using a vacuum pump by watching for diaphragm movement

The EGR valve is not serviceable, except for replacement. To check the valve, proceed as follows:

  1. Connect a tachometer to the engine.
  3. With the engine running at normal operating temperature, with the choke valve fully open, set the engine rpm at 2000. The transmission should be in Park (automatic) or Neutral (manual) with the parking brake On and the wheels blocked.
  5. Disconnect the vacuum hose at the valve. Make sure that vacuum is available at the valve and look at the tachometer to see if the engine speed increases. If it does, the system is OK.
  7. If necessary, replace the valve.


See Figures 2 through 6

Click image to see an enlarged view

Fig. Fig. 2: Remove the air cleaner assembly from the vehicle