Nissan Pick-ups and Pathfinder 1970-1988

Exhaust Gas Recirculation (EGR) System

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OPERATION



See Figures 1, 2, 3, 4, 5 and 6

Gasoline Engines

Exhaust gas recirculation is used to reduce combustion temperatures in the engine, thereby reducing the oxides of nitrogen (NOx) emissions.

An EGR valve is mounted on the center of the intake manifold. The recycled exhaust gas is drawn into the bottom of the intake manifold riser portion through the exhaust manifold heat stove and EGR valve. A vacuum diaphragm is connected to a timed signal port at the carburetor flange.

As the throttle valve is opened, vacuum is applied to the EGR valve vacuum diaphragm. When the vacuum reaches about 2 in. Hg, the diaphragm moves against spring pressure and is in a fully up position at 8 in. Hg of vacuum. As the diaphragm moves up, it opens the exhaust gas metering valve which allows exhaust gas to be pulled into the engine intake manifold. The system does not operate when the engine is idling because the exhaust gas recirculation would cause a rough idle.

On 1974-75 models, an electrically operated solenoid is located in the vacuum line between the EGR valve and the carburetor. The operation of the solenoid is controlled by a temperature sensing switch mounted in the coolant outlet housing. When the temperature of the coolant is below normal operating temperature, the solenoid is electrically activated and blocks the vacuum line leading to the EGR valve, thus preventing exhaust gas recirculation. When the temperature of the engine coolant reaches operating temperature, the solenoid is deactivated and the vacuum is allowed to act upon the EGR valve diaphragm; exhaust gas recirculation takes place.

On 1975-86 models, a thermal vacuum valve inserted in the engine thermostat housing controls the application of vacuum to the EGR valve. When the engine coolant reaches a predetermined temperature, the thermal vacuum valve opens and allows vacuum to be routed to the EGR valve. Below the pre-determined temperature, the thermal vacuum valve closes and blocks vacuum to the EGR valve.

For 1978-80 and all 1986-86 models a Back Pressure Transducer (BPT) valve is installed between the EGR valve and the thermal vacuum valve. The BPT valve has a diaphragm raised or lowered by exhaust back pressure. The diaphragm opens or closes an air bleed, which is connected into the EGR vacuum line. High pressure results in higher levers of EGR, because the diaphragm is raised, closing off the air bleed, which allows more vacuum to reach and open the EGR valve. Thus, the amount of recirculated exhaust gas varies with exhaust pressure.

All 1981-84 models use a Venturi Vacuum Transducer valve (VVT) instead of the BPT valve. The VVT valve monitors exhaust pressure and carburetor vacuum in order to activate the diaphragm which controls the throttle vacuum applied to the EGR control valve. This system expands the operating range of the EGR flow rate as compared to the BPT unit.

All 1978-79 California models have a vacuum delay valve installed in the line between the thermal vacuum valve and the EGR valve. This valve delays rapid drops in vacuum in the EGR line, thus effecting a longer EGR time.

On all 1975 trucks (except Canadian models) and all 1976-77 49 States trucks, the EGR system is equipped with a warning system which monitors the distance the pick-up has traveled and activates a warning light when the EGR system must be checked and possibly serviced. The EGR warning light, mounted on top of the dash, comes on when a predetermined number of miles has been traveled and every time the starter is engaged as a check for a burned out bulb.

To reset the EGR counter, which is mounted on the right fender apron under the hood, remove the grommet installed in the side of the counter and insert the tip of a small screwdriver into the hole. Press down on the knob inside the hole. reinstall the grommet.

Diesel Engines

This system is designed to control the formation of NOx emissions by recirculating the exhaust gas into the intake manifold passage through the control valve.

The EGR flow rate is controlled in three stages in accordance with the engine speed and load. The first stage, High EGR, is obtained through the combination of a closed throttle valve and an open EGR valve. The second stage, Low EGR, is obtained through the opening of the throttle valve. The third stage, Zero EGR, is obtained closing the EGR valve.

The engine load signal is picked up by the potentiometer installed on the injection pump control lever. The engine speed signal is transmitted by an electromagnetic revolution sensor attached to the front cover. The throttle diaphragm and the EGR valve are both actuated by vacuum generated at the vacuum pump. Solenoids are used to convert the electrical signal from the control unit into the vacuum signal.

The EGR system is deactivated under extremely high or low coolant temperatures in order to assure good driveability.



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Fig. Fig. 1: Exhaust gas recirculation system-1975-77



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Fig. Fig. 2: Exhaust gas recirculation system-1978-80



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Fig. Fig. 3: Exhaust gas recirculation system-1981-84



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Fig. Fig. 4: Exhaust gas recirculation system-1985-86 Z20 and Z24 engines



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Fig. Fig. 5: Exhaust gas recirculation system-1986-87 Z24i



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Fig. Fig. 6: Exhaust gas recirculation system-SD25 engines

SERVICE



See Figures 7, 8, 9 and 10

Gasoline Engines
1974-75 VEHICLES
  1. Check the operation of the EGR system as follows:
    1. Visually inspect the entire EGR control system. Clean the mechanism so its free of oil and dirt. Replace any rubber hoses found to be cracked or broken.
    2.  
    3. Make sure that the EGR solenoid valve is properly wired.
    4.  
    5. Increase the engine speed from idling to 3000-3500 rpm. The plate of the EGR control valve diaphragm and the valve shaft should move upward as the engine speed is increased.
    6.  
    7. Disconnect the EGR solenoid valve electrical leads and connect them directly to the vehicle's 12V electrical supply (the battery). Race the engine again with the EGR solenoid valve connected to a 12V power source . The EGR control valve should remain stationary.
    8.  
    9. With the engine running at idle, push up the EGR control valve diaphragm by pressing it up with your finger. When this is done, the engine idle should become rough and uneven.
    10.  

  2.  
  3. Inspect the two components of the EGR system as necessary in the following manner:
    1. Remove the EGR control valve from the intake manifold.
    2.  
    3. Apply 4.7-5.1 in. Hg of vacuum to the EGR control valve by sucking on a tube attached to the outlet on top of the valve. The valve should move to the full up position. The valve should remain open for more than 30 seconds after the application of vacuum is discontinued and the vacuum hose is blocked.
    4.  
    5. Inspect the EGR valve for any signs of warpage or damage.
    6.  
    7. Clean the EGR valve seat with a brush and compressed air to prevent clogging.
    8.  
    9. Connect the EGR solenoid valve to a 12V DC power source and notice if the valve clicks when intermittently electrified. If the valve clicks, it is considered to be working properly.
    10.  
    11. Check the EGR temperature sensing switch by removing it from the engine and placing it in a container of water together with a thermometer. Connect a self-powered test light to the 2 electrical leads of the switch.
    12.  
    13. Heat the container of water.
    14.  
    15. The switch should conduct current when the water temperature is below 77°F (25°C) and stop conducting current when the water reaches a temperature somewhere between 88-106°F (31-41°C). Replace the switch if it behaves otherwise.
    16.  

  4.  

1975-88 VEHICLES
  1. Remove the EGR valve and apply enough vacuum to the diaphragm to open the valve.
  2.  
  3. The valve should remain open for over 30 seconds after the vacuum is removed.
  4.  
  5. Check the valve for damage, such as warpage, cracks, and excessive wear around the valve and seat.
  6.  
  7. Clean the seat with a brush and compressed air and remove any deposits from around the valve and port (seat).
  8.  
  9. To check the operation of the thermal vacuum valve, remove the valve from the engine and apply vacuum to the ports of the valve. The valve should not allow vacuum to pass.
  10.  
  11. Place the Thermal Vacuum valve in a container of water with a thermometer and heat the water. When the temperature of the water reaches 134-145°F (57-63°C), carefully remove the valve and apply vacuum to the ports; the valve should allow vacuum to pass through it.
  12.  
  13. To test the BPT valve, disconnect the two vacuum hoses from the valve. Plug one of the ports. While applying pressure to the bottom of the valve, apply vacuum to the unplugged port and check for leakage. If any exists, replace the valve.
  14.  
  15. To test the check valve installed in some models, remove the valve and blow into the side which connects to the EGR valve. Air should flow. When air is applied to the other side, air flow resistance should be greater. If not, replace the valve.
  16.  
  17. To check the VVT valve, disconnect the top and bottom center hoses and apply vacuum to the top hose. Check for leaks. If a leak is present, replace the valve.
  18.  

Diesel Engines
  1. Visually inspect the entire EGR control system. Clean the mechanism so its free of oil and dirt. Replace any rubber hoses found to be cracked or broken.
  2.  
  3. Remove the EGR valve and apply enough vacuum to the diaphragm to open the valve.
  4.  
  5. The valve should remain open for over 30 seconds after the vacuum is removed.
  6.  
  7. Check the valve for damage, such as warpage, cracks, and excessive wear around the valve and seat.
  8.  
  9. Clean the seat with a brush and compressed air and remove any deposits from around the valve and port (seat).
  10.  
  11. To check the operation of the thermal vacuum valve, remove the valve from the engine and apply vacuum to the ports of the valve. The valve should not allow vacuum to pass.
  12.  
  13. Place the TVV in a container of water with a thermometer and heat the water. When the temperature of the water reaches 134-145°F (57-63°C), remove the valve and apply vacuum to the ports; the valve should allow vacuum to pass through it.
  14.  
  15. To check the VVT valve, block one vacuum port and suck from the other. There should be some initial air leakage which will gradually disappear. If not, replace the valve.
  16.  



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Fig. Fig. 7: Testing the EGR valve-1975-88



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Fig. Fig. 8: Testing the thermal vacuum valve-1975-86



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Fig. Fig. 9: Testing the thermal vacuum valve-1987-88



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Fig. Fig. 10: Cleaning the EGR valve

REMOVAL & INSTALLATION



See Figures 11, 12, 13, 14, 15 and 16

Gasoline Engines
  1. Remove the nuts which attach the EGR tube and/or the Back Pressure (BP) tube to the EGR valve (if so equipped).
  2.  
  3. Unscrew the mounting bolts and remove the heat shield plate from the EGR control valve (if so equipped).
  4.  
  5. Tag and disconnect the EGR vacuum hose(s).
  6.  
  7. Unscrew the mounting bolts and remove the EGR control valve.
  8.  

To install:
  1. Install the EGR valve assembly with mounting bolts to intake manifold location.
  2.  
  3. Connect all vacuum hoses and install the heat shield if so equipped.
  4.  
  5. Connect EGR tube or Back Pressure (BP) tube to the EGR valve if so equipped.
  6.  

Always be sure that the new valve is identical to the old one.

Diesel Engines
  1. Remove the EGR duct and pipe bolts at the side of the control valve.
  2.  
  3. Disconnect the vacuum hose and remove the valve.
  4.  
  5. Install the valve and then connect the pipe and duct. Tighten the bolts to 19-26 ft. lbs. (25-35 Nm).
  6.  



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Fig. Fig. 11: EGR valve-1980



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Fig. Fig. 12: EGR valve-1981



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Fig. Fig. 13: EGR valve-1982-86 Z20, Z22 and Z24 engines



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Fig. Fig. 14: EGR valve-1986-88 Z24i engines



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Fig. Fig. 15: EGR valve-1986-88 VG30i engines



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Fig. Fig. 16: Removing the EGR valve

 
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