Mazda Trucks 1972-1986 Repair Guide

Exhaust Gas Recirculation System

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See Figures 1 and 2

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Fig. Fig. 1: Typical EGR system on trucks without electronic engine control



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Fig. Fig. 2: EGR system used on trucks equipped with electronic engine control

Oxides of nitrogen are formed under conditions of high temperatures and high pressure. By eliminating one of these conditions, the production of NOx is restricted. The exhaust gas recirculation system (EGR) reintroduces a small portion of the exhaust gases into the combustion chamber with the intake charge, thus reducing spark combustion temperature. The EGR system is used on 1976 Mazdas, and all 1977 and later trucks.

Components used through 1978 include an EGR control valve, a three-way solenoid valve, a vacuum amplifier, and a water thermo switch. The control valve, operated by engine vacuum, opens to allow exhaust gases into the intake manifold, and closes to shut them off. The solenoid valve regulates vacuum to the EGR valve, It is governed by the thermo switch. At coolant temperatures below 122ºF (50ºC)131ºF (55ºC) for 1976 onlythe thermo switch is closed. This closes the vacuum passage in the solenoid valve, cutting vacuum to the control valve which prevents exhaust recirculation when the engine is cold. At temperatures above 131ºF (55ºC), the thermo switch opens, allowing intake manifold vacuum to raise the control valve diaphragm, allowing recirculation of exhaust gases. The vacuum amplifier supplies varying amounts of vacuum to the control valve through the solenoid valve, opening or closing it during acceleration or at varying engine speeds.

A slightly different EGR system is used on 1979 and later models. Components include the EGR valve, a water thermo valve, and a vacuum amplifier. The three-way solenoid valve is used on 2.0 liter engines only. The EGR valve is the same vacuum operated unit used in earlier years, and operates in the same manner. The water thermo valve controls the EGR valve, except on California models; on California models, the thermo valve actuates the No. 2 relief valve in the air control valve, and the air control valve regulates the EGR vacuum signal. The thermo valve is closed when coolant temperatures are below 115ºF (46ºC). Above that temperature, the valve opens, allowing vacuum to be transmitted to the EGR valve. The vacuum amplifier performs the same function as in earlier years.

1,970 Mazdas sold in California (1979-80) have two water thermo valves. One is for the EGR system and the other is for the Spark Timing Control System. The thermo valve used in the EGR system has two vacuum hoses; one runs to the vacuum amplifier, and the other runs to the EGR valve.

EGR CONTROL VALVE TEST



See Figures 3, 4, 5, 6, 7 and 8

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Fig. Fig. 3: Some EGR valves may be tested using a vacuum pump by watching for diaphragm movement



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Fig. Fig. 4: Example of a thermal vacuum valve



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Fig. Fig. 5: 1977-78 EGR valve test; 1979 and later models are similar



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Fig. Fig. 6: The EGR valve (arrow) is hidden under vacuum lines and can be tough to reach



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Fig. Fig. 7: Disconnect and label the vacuum hose, and loosen the mounting bolts ...



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Fig. Fig. 8: ... then remove the EGR valve and gasketcarburetor removed for clarity

  1. Start the engine and allow it to idle.
  2.  
  3. On 1976 trucks, disconnect the vacuum hose from the EGR valve. Disconnect the intake manifold vacuum hose from the vacuum amplifier, and connect it to the EGR valve.
  4.  
  5. On 1977-78 trucks, disconnect the vacuum hose which runs from the EGR valve to the three-way solenoid valve at the solenoid valve. Disconnect the intake manifold vacuum hose at the manifold, and connect the EGR valve vacuum hose to the intake manifold fitting.
  6.  
  7. On 1979 and later models, disconnect the vacuum hose from the water thermo valve. The valve is installed in the intake manifold. Disconnect the intake manifold vacuum hose and connect the EGR valve vacuum hose to the intake manifold vacuum fitting.
  8.  
  9. The engine should stall or idle roughly. If it does not, shut off the engine and remove the EGR valve and pipe from the engine. Clean the passages of the valve and pipe with a brush and a wire. Reinstall the parts and repeat the test.
  10.  
  11. If the test is not successful, replace the EGR valve. When engine stall or idle roughness occurs with the manifold vacuum hose connected to the EGR valve, return the hoses to their original positions.
  12.  

THREE-WAY SOLENOID VALVE TEST



See Figure 9

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Fig. Fig. 9: A manifold vacuum gauge can be used to check a variety of vacuum-controlled components

  1. Disconnect the electrical connectors from the thermo switch. Connect a jumper wire between the connectors to simulate a complete circuit.
  2.  
  3. Turn the ignition switch ON.
  4.  
  5. Disconnect the vacuum hose from the EGR valve and blow into the hose. Air should be discharged from the three-way solenoid valve relief port. If it is not, replace the three-way valve.
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  7. Turn the ignition switch off, and remove the jumper wire from the thermo switch connectors. Disconnect the vacuum amplifier vacuum line from the three-way solenoid. Turn the ignition switch back to ON.
  8.  
  9. Blow into the vacuum line disconnected from the EGR valve, and check for air discharge from the vacuum amplifier port on the three-way solenoid valve. If there is no discharge, replace the solenoid valve.
  10.  
  11. After completion of all tests, reconnect the hoses to their original locations.
  12.  

VACUUM AMPLIFIER TEST



  1. Start the engine and warm it to normal operating temperature.
  2.  
  3. Disconnect the vacuum amplifier vacuum hose from the solenoid valve or thermo valve. Connect a vacuum gauge to this hose.
  4.  
  5. Disconnect the vacuum amplifier vacuum hose from the carburetor. The vacuum hose to the intake manifold should remain connected.
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  7. Depress and release the accelerator several times, then allow the engine to idle. The vacuum gauge reading should be 2.0 +- .04 in.Hg.
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  9. Reconnect the vacuum amplifier vacuum hose to the carburetor.
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  11. Increase engine speed to 3,500 rpm. The vacuum gauge reading should be 3.54 in.Hg. If the vacuum amplifier does not test properly, replace it.
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  13. After all tests are completed, return the hoses to their original positions.
  14.  

WATER THERMO VALVE TEST



  1. Drain the cooling system until the coolant level is below the intake manifold. Remove the water thermo valve from the manifold.
  2.  


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

  1. Place a valve in a container of water. Attach a length of vacuum hose to each of the two fittings.
  2.  
  3. Gradually heat the water while observing the temperature.
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  5. By blowing through one of the vacuum hoses, you will be able to tell when the valve opens. The valve should block the passage of air until the water temperature reaches approximately 115ºF (46ºC). If this is not the case, the valve is faulty and must be replaced.
  6.  

 
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