Chevrolet Full-size Cars 1968-1978 Repair Guide

Exhaust Gas Recirculation (EGR) System

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OPERATION





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Fig. Fig. 1 no caption



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Fig. Fig. 2 Differences between the negative backpressure EGR valve (top) and a positive backpressure EGR valve can be found both in the diaphragm and mounting flange



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Fig. Fig. 3 Cross-section of the negative backpressure EGR valve



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Fig. Fig. 4 Cross-section of the positive backpressure EGR valve

All 1973 and later 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 moisture. The exhaust gases lower the combustion temperature, and reduce the amount of oxides of nitrogen (NOx) 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 vehicle accelerates, the carburetor throttle plate uncovers the vacuum port for the EGR valve. At 3-5 in. Hg, the EGR valve opens and then 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 duel 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 vehicles delivered in areas above 4,000 ft. are equipped with back pressure EGR valves. This valve is also used on all 1978 vehicles. 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.

An EGR valve that stays open when it should be closed causes weak combustion, resulting in a rough running engine and/or frequent stalling. Too much EGR flow at idle, cruise, or when cold can cause any of the following:



Engine stopping after a cold start
 
Engine stopping at idle after deceleration
 
Surging during cruising
 
Rough idle
 

An EGR valve which is stuck closed and allows little or no EGR flow causes extreme combustion temperatures (too hot) during acceleration. Spark knock (detonation or pinging), engine over heating and excess engine emissions can all be a result, as well as engine damage. See the accompanying EGR system diagnosis chart for possible cause and correction procedures.

REMOVAL & INSTALLATION



EGR Valve


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Fig. Fig. 5 Some EGR valves may be tested using a vacuum pump by watching for diaphragm movement



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Fig. Fig. 6 Common EGR mounting location on V8 engines



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Fig. Fig. 7 Disconnect the vacuum hose going to the EGR valve



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Fig. Fig. 8 Remove the attaching bolts which retain the EGR valve to the intake manifold



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Fig. Fig. 9 Lift the EGR valve off of the engine



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Fig. Fig. 10 Discard the old EGR gasket. Use a new gasket when installing the EGR valve to the engine



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Fig. Fig. 11 Example of a thermal vacuum valve-Chevrolet shown

  1. Detach the vacuum lines from the EGR valve.
  2.  
  3. Unfasten the two bolts or bolt and clamp which attach the valve to the manifold. Withdraw the valve.
  4.  
  5. Installation is the reverse of removal. Always use a new gasket between the valve and the manifold. On dual diaphragm valves, attach the 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.
  6.  

TVS Switch


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Fig. Fig. 12 A common Thermostatic Vacuum Switch (TVS). The number of vacuum connections may vary by year and engine application

  1. Drain the radiator.
  2.  


CAUTION
When draining the coolant, keep in mind that cats and dogs are attracted by the ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.

  1. Disconnect the vacuum lines from the switch noting their locations. Remove the switch.
  2.  
  3. Apply sealer to the threaded portion of the new switch, and install it, torquing to 15 ft. lbs.
  4.  
  5. Rotate the head of the switch to a position that will permit easy hookup of vacuum hoses. Then install the vacuum hoses to the proper connectors.
  6.  

EGR VALVE CLEANING



Valves That Protrude from Mounting Face


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Fig. Fig. 13 To clean the EGR valve, tap the protruding valve tip lightly with a hammer


WARNING
Do not wash the valve assembly in solvents or degreasers. Permanent damage to the valve diaphragm may result.

  1. Remove the vacuum hose from the EGR valve assembly. Remove the two attaching bolts, remove the EGR valve from the intake manifold and discard the gasket.
  2.  
  3. Holding the valve assembly in hand, tap the valve lightly with a small plastic hammer to remove exhaust deposits from the valve seat. Shake out any loose particles. DO NOT put the valve in a vise.
  4.  
  5. Carefully remove any exhaust deposits from the mounting surface of the valve with a wire wheel or putty knife. Do not damage the mounting surface.
  6.  
  7. Depress the valve diaphragm and inspect the valve seating areas through the valve outlet for cleanliness. If the valve and/or seat are not completely clean, repeat Step 2.
  8.  
  9. Look for exhaust deposits in the valve outlet, and remove any deposits with an old screwdriver.
  10.  
  11. Clean the mounting surfaces of the intake manifold and valve assembly. Using a new gasket, install the valve assembly to the intake manifold. Torque the bolts to 25 ft. lbs. Connect the vacuum hose.
  12.  

Shielded Valves or Valves That Do Not Protrude
  1. Clean the base of the valve with a wire brush or wheel to remove exhaust deposits from the mounting surface.
  2.  
  3. Clean the valve seat and valve in an abrasive-type spark plug cleaning machine or sandblaster. Most machine shops provide this service. Make sure the valve portion is cleaned (blasted) for about 30 seconds, and that the valve is also cleaned with the diaphragm spring fully compressed (valve unseated). The cleaning should be repeated until all deposits are removed.
  4.  
  5. The valve must be blown out with compressed air thoroughly to ensure all abrasive material is removed from the valve.
  6.  
  7. Clean the mounting surface of the intake manifold and valve assembly. Using a new gasket, install the valve assembly to the intake manifold. Torque the bolts to 25 ft. lbs. Connect the vacuum hose.
  8.  

 
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