Exhaust Gas Recirculation: Description and Operation

Description

Fig. 11 - Typical EGR System:

This system, Fig. 11, used on 1976 California vehicles equipped with a manual transmission and all 1977-82 models, introduces small amounts of exhaust gas into the combustion cycle, reducing combustion temperatures, in turn reducing the generation of nitrous oxides (NOx). The amount of exhaust gas recirculated and the timing of the cycle are controlled by various parameters such as engine vacuum and temperature.

Operation

The EGR valve is a vacuum operated, cone type unit. When the valve is open, exhaust gas enters the intake manifold. A three-way solenoid valve is used to control the vacuum supply to the EGR valve. Power to the three-way solenoid is controlled by a water thermo switch. When the water temperature is below 131°F., the switch is closed, in turn energizing the three-way solenoid valve. The solenoid valve then closes the vacuum passage within the valve and cutting vacuum to the EGR valve. When water temperature is above 131°F., the water thermo switch is open, deenergizing the three-way solenoid valve. The vacuum passage within the valve is open, permitting intake manifold vacuum to control the EGR valve diaphragm. A vacuum amplifier is used to supply varying amounts of vacuum to the EGR valve diaphragm to actuate the valve during acceleration or at various engine speeds.

A water thermo valve is used on 1979-81 models to control EGR operation according to engine temperature. When engine coolant temperature is below 115°F, the vacuum supply is closed off to the EGR valve. When engine coolant temperature is above 115°F, the water thermo valve opens and supplies vacuum to actuate the EGR valve. When the warning lamp is lit, the EGR valve should be tested and the switch reset.