Exhaust Gas RECIRCULATION System: Operation

Exhaust gases flow out of exhaust manifold, through EGR tube into rear of balance tube then into EGR control valve. From EGR valve, gases flow through center of balance tube to be distributed to front and rear intake manifold. Exhaust gases passing through balance tube are cooled by engine coolant. Flow of exhaust gases are controlled by a control valve which is operated by carburetor vacuum. At idle, vacuum is insufficient to operate control valve and exhaust gases are not recirculated. When throttle valve of carburetor opens, vacuum is increased and exhaust gases begin to recirculate. However, when throttle is opened fully and vacuum drops below 2 in. Hg (50.8 mm Hg) valve will again close. To improve starting ability and driveability of engine in cold weather, a thermo switch, located in passenger compartment, operates solenoid valve on EGR control valve. When passenger compartment temperature is below 30°F (-1 °C), current flows through solenoid and shuts off vacuum passage. This prevents exhaust gases from recirculating. When temperature is over 52°F (11°C) current does not flow through solenoid and exhaust gas is allowed to recirculate.