GM Firebird 1967-1981 Repair Guide

Exhaust Gas Recirculation (EGR)


See Figures 1 and 2

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Fig. Fig. 1: Cross-sectional view of a negative backpressure EGR valve

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Fig. Fig. 2: Cross-sectional view of a positive backpressure EGR valve

This system was used on 1973-81 Models.


All 1973-81 engines are equipped with Exhaust Gas Recirculation (EGR). This system consists of a metering valve, a vacuum line to the carburetor, and cast-in exhaust gas passages in the intake manifold. The EGR valve is controlled by carburetor vacuum, and accordingly opens and closes to admit exhaust gases into the fuel/air mixture. The exhaust gases lower the combustion temperature, and reduce the amount of oxides of nitrogen (NO;zx) produced. The valve is closed at idle between the two extreme throttle positions.

In most installations, vacuum to the EGR valve is controlled by a Thermal Vacuum Switch (TVS); the switch, which is installed into the engine block, shuts off vacuum to the EGR valve until the engine is hot. This prevents the stalling and lumpy idle which would result if EGR occurred when the engine was cold.

As the car accelerates, the carburetor throttle plate uncovers the vacuum port for the EGR valve. At 3-5 in. Hg, the EGR valve opens and some of the exhaust gases are allowed to flow into the air/fuel mixture to lower the combustion temperature. At full-throttle the valve closes again.

Some California engines are equipped with a dual diaphragm EGR valve. This valve further limits the exhaust gas opening (compared to the single diaphragm EGR valve) during high intake manifold vacuum periods, such as high-speed cruising, and provides more exhaust gas recirculation during acceleration when manifold vacuum is low. In addition to the hose running to the thermal vacuum switch, a second hose is connected directly to the intake manifold.

For 1977, all California models and cars delivered in areas above 4000 ft. are equipped with backpressure EGR valves. This valve is also used on all 1978-81 models. The EGR valve receives exhaust back pressure through its hollow shaft. This exerts a force on the bottom of the control valve diaphragm, opposed by a light spring. Under low exhaust pressure (low engine load and partial throttle), the EGR signal is reduced by an air bleed. Under conditions of high exhaust pressure (high engine load and large throttle opening), the air bleed is closed and the EGR valve responds to an unmodified vacuum signal. At wide open throttle, the EGR flow is reduced in proportion to the amount of vacuum signal available.

1979 and later models have a ported signal vacuum EGR valve. The valve opening is controlled by the amount of vacuum obtained from a ported vacuum source on the carburetor and the amount of backpressure in the exhaust system.


See Figures 3 through 7

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Fig. Fig. 3: EGR mounting position on inline 6-cylinder engines

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Fig. Fig. 4: The EGR valve used on 8-cylinder engines is mounted on the right-hand side of the carburetor on the intake manifold

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Fig. Fig. 5: To remove the EGR valve, first detach the vacuum line ...

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Fig. Fig. 6: ... then remove the mounting bolts and lift the valve off of the intake manifold

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Fig. Fig. 7: Make sure to remove the old gasket and replace it with a new one 305 and 350 engines shown

  1. Detach the vacuum lines from the EGR valve.
  3. Unfasten the two bolts or bolt and clamp which attach the valve to the manifold. Withdraw the valve.

On turbocharged models, the EGR valve is attached to the plenum.

  1. Installation is the reverse of removal. Always use a new gasket between the valve and the manifold. On dual diaphragm valves, attach carburetor vacuum line to the tube at the top of the valve, and the manifold vacuum line to the tube at the center of the valve.

TVS Switch

  1. Drain the radiator.
  3. Disconnect the vacuum lines from the switch noting their locations. Remove the switch.
  5. Apply sealer to the threaded portion of the new switch, and install it, tightening it to 15 ft. lbs.
  7. Rotate the head of the switch to a position that will permit easy hookup of vacuum hoses. Then install vacuum hoses to the proper connectors.