Exhaust Gas Recirculation: Description and Operation

The exhaust gas recirculation (EGR) system reduces the formation of NOx emissions by reducing the peak combustion temperatures. The system injects exhaust gasses into the intake manifold, reducing the air/fuel mixture charge to the cylinder. With a reduced amount of combustable mixture in the charge, combustion temperatures are reduced.

Fig. 7 - Exhaust Gas Recirculation System:

The system consists of an EGR control valve, which controls the amount of exhaust gas recirculated, a pickup for exhaust gasses in the exhaust manifold and a connection at the intake manifold for introduction of exhaust gas, and a vacuum control system which includes a ported vacuum tap and a thermovalve. On 1979-80 2000 engines, exhaust gasses are routed internally through the cylinder head to the EGR valve, and from the EGR valve to the plenum of the intake manifold. On other models, routing of exhaust gasses is done externally, Fig. 7.

Exhaust gasses are routed from the exhaust manifold to the EGR valve. The EGR valve is controlled by a ported vacuum signal, which causes the valve to open and close. The vacuum signal is taken from a port above the throttle plate in the carburetor. The vacuum signal is routed through a thermovalve, which blocks the signal until the engine coolant reaches specified temperature.

When the engine is at operating temperature, vacuum is applied to the EGR valve as the throttle is opened. The combination of vacuum signal and exhaust back pressure causes the EGR valve to open, allowing exhaust gasses to be routed to the delivery port in the intake manifold. Due to positioning of the vacuum signal port, recirculation of exhaust gasses is greatest under high engine loads, and no vacuum signal is applied to the valve at idle.