EGR Valve: Description and Operation

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).

Fig. 39 EGR connections. 4 & 6 cyl. engines:

On four and 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, Fig. 39.


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. 40 Venturi vacuum amplifier:

A Venturi Vacuum Amplifier, Fig. 40, 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.


There are three basic types of EGR valves: the ported type, the back pressure type and the electronic-sonic type.
The electronic-sonic type valve is used with the EEC system. Information on this valve can be found in the ``EEC'' section of this chapter.

Fig. 41 Ported type EGR valves:

Ported type valves, Fig. 41, may be of the poppet or tapered stem design and can have base or side entry. The two passages connecting the exhaust system to intake manifold are blocked by a valve which is opened by vacuum and closed by spring pressure.

Fig. 42 EGR valve with integral back pressure transducer:

Two types of back pressure valves are used. The integral back pressure transducer valve, Fig. 42, cannot be opened by vacuum until the bleed hole is closed by exhaust back pressure. When open, the valve oscillates at a level dependent on the exhaust back pressure flowing through the orifice. Valve opening increases as signal vacuum and exhaust back pressure increases.

Fig. 43 EGR valve with remote back pressure transducer:

Some vehicles use an EGR valve with a remote back pressure transducer, Fig. 43. This assembly operates the same as the EGR valve with integral back pressure transducer valve and is serviced only as an assembly.