System Description

Fig.42 - Spacer Entry EGR Connection:

In this system the exhaust gases are metered through the EGR valve to a passage in the carburetor spacer thereby diluting the air fuel mixture entering the combustion chambers. Dilution of the incoming mixture lowers peak flame temperatures during combustion and thus limits the formation of nitrogen oxides (NOx).

Most eight cylinder engines use the "Spacer Entry" EGR System which has the EGR valve mounted on the rear of the carburetor spacer, Fig. 42. The exhaust gases are taken from a drilled passage in the exhaust crossover of the intake manifold. The exhaust gas is then routed through a metered EGR valve to a passage in the carburetor spacer and fed into the primary bore.

Some 1974 eight cylinder engines use the "Floor Entry" EGR system, which has the EGR valve mounted on the rear of the intake manifold. The EGR valve controls the exhaust gases that enter specially cast passages in the manifold from the exhaust crossover passage. When the valve opens, the exhaust crossover is then opened to the two drilled passages in the floor of the intake manifold riser under the carburetor.

On six cylinder engines, the EGR system is basically the same as the Spacer Entry EGR System except that exhaust gas is routed directly from the exhaust manifold.

Two variables control the operation of the EGR system, 1) engine coolant temperature and 2) carburetor vacuum. When engine coolant temperature is below the specified level the EGR system is locked out by a temperature controlled vacuum switch. The vacuum switch is installed in series with the EGR valve. This valve receives vacuum from a port in the carburetor body. When the valve is closed due to lower coolant temperature, no vacuum is applied to the EGR valve and no exhaust gas is fed to the air-fuel mixture. When the engine coolant temperature reaches the specified level, the valve opens allowing vacuum to be applied to the EGR valve. Exhaust gas is then fed to the air-fuel mixture.

The second factor controlling EGR operation is carburetor vacuum. The location of the EGR port in the carburetor determines at what point vacuum is sent to the EGR valve. Vacuum should be fed to the EGR vacuum control valve when the primary throttle plate reaches a position corresponding to a road speed of approximately 20 mph under light acceleration.

Fig.43 - Venturi Vacuum Amplifier:

Fig.44 - Venturi Vacuum Amplifier:

A Venturi Vacuum Amplifier, Figs. 43 and 44, uses a weak venturi-vacuum signal to produce a strong intake manifold vacuum to operate the EGR valve, thereby achieving an accurate, repeatable and almost exact proportion between venturi airflow and EGR flow. This assists in controlling oxides of nitrogen with minimal sacrifice in driveability.

Fig.20 - Dual Area Diaphragm:

Dual Area Diaphragms - On 1973-80 vehicles, dual-area diaphragms are used, Fig. 20. These diaphragms offset effects of engines using the EGR system and equipped with automatic transmissions. The new diaphragms permit vehicles to function with satisfactory shift spacing and shift feel.

To test, remove the vacuum diaphragm and test unit using an outside vacuum source. Set regulator on tester to 18 in. Hg with end of vacuum hose blocked off then connect vacuum hose to vacuum diaphragm unit. If unit does not hold 18 in. Hg reading, the diaphragm is leaking and must be replaced.