GM Full-Size Trucks 1970-1979 Repair Guide

Exhaust Gas Recirculation


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, part-throttle, and wide open throttle operation. In 1974, a thermo-delay switch was added to delay operation of the valve during engine warm-up, when NOx levels are already at a minimum.

There are actually three types of EGR systems: Vacuum Modulated, Positive Exhaust Backpressure Modulated, and Negative Exhaust Backpressure Modulated. The principle of all the systems is the same; the only difference is in the method used to control how far the EGR valve opens.

In the Vacuum Modulated system, which is used on all trucks through 1976, and some models thereafter, the amount of exhaust gas admitted into the intake manifold depends on a ported vacuum signal. A ported vacuum signal is one taken from the carburetor above the throttle plates. Thus, the vacuum signal (amount of vacuum) is dependent on how far the throttle plates are opened. When the throttle is closed (idle or deceleration) there is no vacuum signal. Thus, the EGR valve is closed, and no exhaust gas enters the intake manifold. As the throttle is opened, a vacuum is produced, which opens the EGR valve, admitting exhaust gas into the intake manifold.

In the Exhaust Backpressure Modulated system, a transducer is installed in the EGR valve body, reacting to either positive or negative backpressure, depending on design. The vacuum used is still ported vacuum, but the transducer uses exhaust gas backpressure to control an air bleed within the valve to modify this vacuum signal. Backpressure valves are used on all light duty emissions California and High Altitude engines in 1976 and 1978, and on most engines in 1979. The choice of either a positive or negative backpressure valve is determined by measurement of the engine's normal backpressure output. Negative valves are used on engines with relatively low backpressure; positive valves are used on engines with relatively high backpressure. The choice of valve usage is made at the factory, and is nothing for the backyard mechanic to worry about; however, if the valve is replaced, it is important to install the same type as the original. The difference between the three valves (ported, positive, or negative) can be determined by the shape of the diaphragm plate; your Chevrolet or GMC dealer will be able to match the old valve to a new one.

On 6-cylinder engines, the EGR valve is located on the intake manifold adjacent to the carburetor. On small block V8 engines, the valve is located on the right rear side of the intake manifold adjacent to the rocker arm cover. Mark IV V8 EGR valves are located in the left front corner of the intake manifold in front of the carburetor.


The EGR valve is not serviceable, except for replacement. To check the ported vacuum signal 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 2,000. 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, a malfunction of the valve is indicated.
  7. If necessary, replace the valve.

A back pressure EGR valve is used on all light duty emissions California and High Altitude 6-cylinder and V8 engines in 1977 and 1978, and most 1979 models.

The system can be tested as follows:

  1. Remove air cleaner so that the EGR valve diaphragm movement can be observed. The choke secondary vacuum break TVS can be unclipped and removed from the air cleaner body, rather than removing hoses.
  3. Plug the intake manifold air cleaner vacuum fitting. Connect as tachometer.
  5. Start the engine and warm to operating temperature. Open the throttle part way and release. Watch or feel the EGR diaphragm for movement. The valve should open slightly when the throttle is opened and close when it is released.
  7. Remove the EGR hose from the EGR valve and plug the hose. Place the carburetor cam follower on the second step of the fast idle cam and note the speed.
  9. Attach a vacuum hose between the air cleaner vacuum fitting and the EGR valve. Note the speed change. The speed should drop at least 200 rpm with automatic transmissions, or at least 150 with manuals.

If the EGR valve does not meet the criteria specified in these tests, it must be replaced.