EGR CONTROL VALVE TEST

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

Fig. 2: Example of a thermal vacuum valve

Fig. 3: 1977–78 EGR valve test; 1979 and later models are similar

Fig. 4: The EGR valve (arrow) is hidden under vacuum lines and can be tough to reach

Fig. 5: Disconnect and label the vacuum hose, and loosen the mounting bolts . . .

Fig. 6: . . . then remove the EGR valve and gasket — carburetor removed for clarity

Start the engine and allow it to idle.
On 1976 trucks, disconnect the vacuum hose from the EGR valve. Disconnect the intake manifold vacuum hose from the vacuum amplifier, and connect it to the EGR valve.
On 1977–78 trucks, disconnect the vacuum hose which runs from the EGR valve to the three-way solenoid valve at the solenoid valve. Disconnect the intake manifold vacuum hose at the manifold, and connect the EGR valve vacuum hose to the intake manifold fitting.
On 1979 and later models, disconnect the vacuum hose from the water thermo valve. The valve is installed in the intake manifold. Disconnect the intake manifold vacuum hose and connect the EGR valve vacuum hose to the intake manifold vacuum fitting.
The engine should stall or idle roughly. If it does not, shut off the engine and remove the EGR valve and pipe from the engine. Clean the passages of the valve and pipe with a brush and a wire. Reinstall the parts and repeat the test.
If the test is not successful, replace the EGR valve. When engine stall or idle roughness occurs with the manifold vacuum hose connected to the EGR valve, return the hoses to their original positions.