GM Full-Size Trucks 1988-1998 Repair Information

Electric Fuel Pump

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See Figure 1

All Chevrolet/GMC fuel-injected vehicles that use the MFI system are equipped with an electric fuel pump. For a fuel injection system to work properly, the pump must develop pressures well above those of a mechanical fuel pump. This high pressure is maintained within the lines even when the engine is off. Extreme caution must be used to safely release the pressurized fuel before any work is begun.


CAUTION
Always relieve the fuel pressure within the system before any work is begun on any fuel component. Failure to safely relieve the pressure may result in fire and/or serious injury.



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Fig. Fig. 1: Exploded view of the MFI fuel supply system

REMOVAL & INSTALLATION



Single Tank

See Figures 2, 3, 4 and 5

  1. Disconnect the negative battery cable.
  2.  
  3. Relieve the fuel system pressure. Refer to the fuel system relief procedure in this section.
  4.  
  5. Raise the vehicle and support it safely with jackstands.
  6.  
  7. Drain the fuel system and remove the fuel tank. Refer to the fuel tank removal procedure in this section.
  8.  
  9. Remove the fuel sender assembly turning it counterclockwise using tool J-39765 or its equivalent.
  10.  
  11. Remove the assembly from the tank.
  12.  
  13. Pull the pump up into the attaching hose while pulling outward from the bottom support. Do not damage the rubber insulator or the strainer.
  14.  
  15. Inspect the fuel pump attaching hose and the rubber sound insulation for signs of deterioration.
  16.  
  17. Inspect the strainer for blockage and damage.
  18.  



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Fig. Fig. 2: A special tool is usually available to remove or install the fuel pump locking cam



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Fig. Fig. 3: A brass drift and a hammer can also be used to loosen the fuel pump locking cam



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Fig. Fig. 4: Once the locking cam is released, it can be removed to free the fuel pump



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Fig. Fig. 5: Once the locking cam is removed, the fuel pump assembly can be lifted straight up and out of the tank

To install:
  1. Install the fuel pump assembly into the attaching hose.
  2.  

Be careful not to bend or fold over the fuel strainer when installing the fuel sender as this will restrict fuel flow.

  1. Install the fuel sender into the fuel tank. Insert a new O-ring seal.
  2.  
  3. Install the camlock assembly turning it clockwise to lock it.
  4.  
  5. Install the fuel tank. Refer to the fuel tank installation procedure in this section.
  6.  
  7. Connect the negative battery cable.
  8.  
  9. Turn the ignition switch ON for 2 seconds then turn the switch OFF for 10 seconds. Again, turn the ignition switch ON and check for leaks.
  10.  

Dual Tanks
  1. Disconnect the negative battery cable.
  2.  
  3. Loosen the filler cap(s) to relieve fuel tank pressure.
  4.  
  5. Disconnect the fuel pipes from the pump.
  6.  
  7. Slide the pump out of the bracket.
  8.  

To install:
  1. Install new fuel pipe O-rings.
  2.  
  3. Position the pump in the pump bracket.
  4.  
  5. Connect the fuel feed pipe and suction pipe to the fuel pump.
  6.  
  7. Using a backup wrench to stop the fuel pump from turning, tighten the fittings to 22 ft. lbs. (30 Nm).
  8.  
  9. Engage the electrical connector.
  10.  
  11. Connect the negative battery cable.
  12.  
  13. Turn the ignition switch ON for 2 seconds then turn the switch OFF for 10 seconds. Again, turn the ignition switch ON and check for leaks.
  14.  

TESTING



Fuel Pump Relay
  1. Turn the ignition OFF .
  2.  
  3. Remove the relay.
  4.  
  5. Locate the two terminals on the relay, which are connected to the coil windings. Check the relay coil for continuity. Connect the common meter lead to terminal 85 and positive meter lead to terminal 86. There should be continuity. If not, replace the relay.
  6.  

Fuel Pump
  1. Turn the ignition key OFF for ten seconds.
  2.  
  3. Turn the ignition key to the ON position but do not start the engine.
  4.  
  5. You should here the fuel pump operate for two seconds (a light electrical humming sound).
  6.  
  7. Apply a 12 volt fused jumper wire to the fuel pump prime (test) connector. This should be located in the driver's side of the engine compartment. The fuel pump should operate.
  8.  
  9. If the fuel pump does operate, remove and test the relay. If the relay is defective, replace the relay and repeat the test.
  10.  
  11. If the relay is operating properly, connect a grounded test light to the fuel pump relay control wire (green with white stripe) at the relay. Turn the ignition switch to the ON position. The test light should illuminate for two seconds and go out. If the test light does not illuminate, check the relay control wire for continuity and repair as necessary, then repeat the test.
  12.  
  13. Connect a grounded test light to the feed wire (orange) at the relay and turn the ignition key ON . The test light should illuminate. If the light does not illuminate, test feed wire for continuity and the fuse in the underhood fuse relay center. Repair as necessary and repeat the test.
  14.  
  15. If the fuel pump is still not functioning, connect a fused jumper wire between the feed (orange) wire and the fuel pump motor (gray) wire at the relay. The pump should operate.
  16.  
  17. If the pump does not operate, check the fuel pump motor wire for continuity and repair as necessary.
  18.  
  19. If the pump will still not operate, replace the pump.
  20.  
  21. If the pump does operate, start the vehicle and remove the relay. The vehicle should continue to run. If the vehicle stalls, check the fuel pump/oil pressure switch and its wiring. Repair as necessary and repeat the test.
  22.  

 
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