Exhaust Gas Recirculation: Description and Operation

Fig.14 - Exhaust Gas Recirculation System:

Fig.15 - Exhaust Gas Recirculation System:

Fig.16 - Exhaust Gas Recirculation System:

Fig.16a - Exhaust Gas Recirculation System:

This system, Figs. 14, 15, 16 and 16A, is used to reduce oxides of nitrogen emissions by lowering the peak combustion temperature in the engine. This is accomplished by introducing a metered amount of exhaust gases into the intake manifold at throttle positions other than idle.

Fig.17 - Exhaust Gas Recirculation (EGR) Valve:

The EGR valve, Fig. 17, mounted on the intake manifold is used to control the amount of recirculated gas. The recirculated gases are drawn into the bottom of the intake manifold riser, through the exhaust manifold heat stove and to the EGR valve. The EGR vacuum diaphragm is connected to a timed signal port at the carburetor flange.

As, the carburetor throttle valve is opened, vacuum is applied to the EGR valve diaphragm. When vacuum reaches about 3.5 in. Hg., the EGR diaphragm starts to open and exhaust gas recirculation starts. At about 8 in. Hg. the EGR valve is fully open.

A thermal vacuum valve, used on 1976-82 models, senses engine coolant temperature to regulate the EGR system.

On 1976-78 models, a 1 18-126"F thermal vacuum valve is used. On 1979-82 models, a 115 to 12O° F vacuum thermal valve is used. When coolant temperature is below the specified range, the EGR system will not operate. When coolant temperature is above the specified range, the vacuum thermal valve will allow the signal vacuum to be routed to the EGR valve and the EGR system will operate.

On some 1979-82 models a back pressure transducer is used. When exhaust back-pressure is relatively high, as during acceleration and some cruising conditions, exhaust back- pressure traveling through the metal tube overcomes spring tension on the diaphragm within the back-pressure transducer valve, and closes the valve atmospheric vent.

With the back-pressure transducer valve no longer vented to atmosphere, the vacuum signal now passes through the back-pressure transducer valve and the EGR valve. When vacuum signals the EGR valve, exhaust gas recirculation commences.

When exhaust back-pressure is too low to overcome diaphragm spring tension, the vacuum signal is vented to atmosphere and does not pass through to the EGR valve. With no vacuum signal applied to the EGR valve, exhaust gas does not recirculate.

The vacuum control valve used on 1979-81 models except California, controls the vacuum signal from the carburetor to the EGR valve. When manifold vacuum drops below a specified value, the vacuum control valve opens and vents the EGR valve ported signal vacuum to the atmosphere. When intake manifold is above the specified value, the vacuum control valve closes and allows the vacuum signal to be applied to the EGR valve.