See Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11
Oxides of nitrogen can only be formed under conditions of high pressure and high temperature. Elimination of one of these conditions reduces their production. A reduction of peak combustion temperature is accomplished by exhaust gas recirculation into the carburetor.
1974 California models, and all 1975 and later models (1988) are equipped with an EGR system. 1974 18R-C engines have a tube running from the exhaust manifold to the EGR valve, and a tube from the valve to the carburetor above the throttle plates. 1975-80 20R engines and 2F engines are similar, with the addition of an exhaust gas cooler cast into rear of the cylinder head. 1974-76 systems use a thermo switch, computer speed sensor, EGR valve and a vacuum switching valve (VSV). In addition, 1974 18R-C engines have a temperature switch at the carburetor flange. When the speed sensor and thermo switch are both within their operating ranges, the computer turns the VSV on, which in turn, opens the EGR valve, allowing a proportion of the exhaust gases to re-enter the engine through the carburetor. 1977 and later years, EGR systems eliminates all but the EGR valve, and add a TVSV (thermostatic VSV). At coolant temperatures above 50°F, the TSVS opens allowing engine vacuum to open the EGR valve. This permits a recirculation of exhaust gases.
See Figures 12, 13 and 14EGR Valve Check
- Allow the engine to warm up and remove the top from the air cleaner.
Do not remove the entire air cleaner assembly.
- Disconnect the hose (white tape coded), which runs from the vacuum switching valve to the EGR valve, at its EGR valve end.
- Remove the intake manifold hose (red coded) from the vacuum switching valve and connect it to the EGR valve. When the engine is at idle, a "hollow" sound should be heard coming from the air cleaner.
- Disconnect the hose from the EGR valve; the hollow sound should disappear.
- If the sound doesn't vary, the EGR valve is defective and must be replaced.
- Reconnect the vacuum hoses as they were originally found. Install the top of the air cleaner.
- Start the engine.
- Disconnect the vacuum hose leading from the EGR valve.
- Disconnect the hose coming from the intake manifold and connect it to the empty pipe on the EGR valve.
- When applying vacuum directly to the EGR valve, the engine should stall, if not, the EGR valve will probably require replacement.
- Disconnect the electrical lead which runs to the EGR valve thermo-sensor.
- Remove the thermo-sensor from the side of the EGR valve.
- Heat the thermo-sensor in a pan of water to the following temperature: 260°F.
- Connect an ohmmeter, in series with a 10- resistor, between the thermo-sensor terminal and case.
- With the ohmmeter set on the k- scale, the following reading should be obtained: 2.55K-
- Replace the thermo-sensor if the ohmmeter readings vary considerably from those specified.
- To install the thermo-sensor on the EGR valve, tighten it to 15-21 ft. lbs.
Checking the EGR Vacuum Modulator
- Tag and disconnect all hoses leading form the vacuum modulator.
- Remove the vacuum modulator.
- Unscrew the vented top plate and remove the filter.
- Check the filter for any contamination or other damage.
- Clean the filter using compressed air.
- Installation is in the reverse order of removal.
1975 trucks have an EWGR warning light on the instrument panel above the windshield wiper switch. The light remains on until the switch is reset. The switch should not be reset until all required maintenance on the EGR and other emission systems has been performed. To reset the switch:
- Remove the lockscrew from the switch cover. This screw requires a tool with two prongs.
- Remove the cover. Reset the switch by moving it to the position opposite its present position.
- Replace the cover and lockscrew, and check to see that the light turns on when the ignition switch is turned to the Start position.