Exhaust Gas Recirculation: Description and Operation

The EGR system recirculates a portion of the exhaust gasses to dilute the air-fuel mixture, lowering the combustion temperature and reducing oxides of nitrogen in the exhaust gasses. The system basically consists of an EGR valve and a thermo valve.

Vacuum applied to the EGR valve is controlled by the thermo valve which senses coolant temperature. The thermo valve closes the EGR valve when coolant temperature is lower than a pre-set value to aid initial start-up and driveability. The EGR valve is also closed at idle since engine vacuum is low.

Fig. 11 - EGR System:

On 1978-82 models, a dual EGR valve which consists of a primary and secondary valves which are controlled by differences in carburetor vacuum in response to throttle opening. The primary valve controls EGR flow when the throttle is slightly opened, while the secondary control valve allows recirculating exhaust gasses to enter the intake manifold when the throttle is further opened.