Exhaust Gas RECIRCULATION System: Operation
On 1974-76 models, exhaust gases flow out of exhaust manifold through EGR tube into EGR valve. From EGR valve, the exhaust gases enter the intake manifold. The EGR valve is controlled by intake vacuum, coolant temperature switch (thermal vacuum valve), and EGR solenoid (if equipped). When coolant temperature is below a specified range, the EGR system is not operational. When coolant temperature is above 145°F (63°C) on most models, coolant temperature switch and EGR solenoid are off, allowing manifold vacuum to reach EGR valve.
On 1977-79 models, exhaust gases flow out of exhaust manifold through EGR tube and into EGR control valve. The BPT valve, if equipped, monitors exhaust pressure in order to control intake manifold vacuum applied to EGR control valve. See Fig 1. The EGR control valve regulates actual amount of exhaust gas to be recirculated into the intake manifold. Operation of the EGR control valve is directed by the thermal vacuum valve which operates on carburetor vacuum and engine coolant temperature. The EGR system does not operate at idle or under full throttle (heavy load) conditions, when coolant temperature is low (thermal vacuum valve closed), or when exhaust pressure is low.