GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC P42 Commercial Chassis 🢒 Engine 🢒 Engine Controls - 6.5L (L57) 🢒 Fuel Supply System Check

Fuel Supply System Check EFI/MFI

If the fuel supply system is not delivering enough fuel, or if air is being drawn into the fuel injection system, the driveability could be greatly effected or a Cranks But Will Not Run symptom could exist. If another diagnosis indicates or if the fuel supply system is suspected of not delivering enough fuel or drawing air, the following systems should be checked.

    • Air leaks or restrictions on the suction side of the fuel pump will seriously affect the pump output.
    • Restriction in the fuel return system.
    • Make certain that there is sufficient fuel in the tank.
    • Check for leaks at all of the fuel connections from the fuel tank to the injection pump.
    • Tighten any loose connections.
    • With the engine running, check all of the hoses and the lines for flattening or kinks that would restrict the flow of fuel.

Fuel Lift Pump Flow Check
  1. Remove the FUEL SOL fuse (L57 EFI) or the Engine Shutoff Solenoid connector on the injection pump (L57 MFI).
  2. Disconnect the pipe at the lift pump outlet fitting.
  3. Connect a hose at the lift pump outlet fitting and place a 1 liter (0.946 quart) container at the hose in order to collect the fuel.
  4. Crank the engine and measure the amount of fuel.
    5. • If there is more than 0.24 liter (½ pint) in 15 seconds, refer to Fuel Lift Pump Pressure Check.
    • If there is less than 0.24 liter (½ pint) in 15 seconds, refer to Fuel Lift Pump Suction Line Check.

Fuel Lift Pump Suction Line Check
  1. Remove the fuel tank cap and repeat the Lift Pump Flow Check.
    2. • If the flow is more than 0.24 liter (½ pint) in 15 seconds, replace the defective fuel tank cap.
    • If the flow is less than 0.24 liter (½ pint) in 15 seconds, go to the next step.
  2. Separate the lift pump suction line from the fuel sender.
  3. Use an additional hose in order to connect the suction line to a source of clean fuel.
  4. Repeat the Lift Pump Flow Check.
    5. • If the flow is more than 0.24 liter (½ pint) in 15 seconds, remove the fuel sender and check for a restriction.
    • If the flow is less than 0.24 liter (½ pint) in 15 seconds, Go to Step 5.
  5. Check the lift pump suction line for a restriction.
    6. • If a restriction exists, repair and recheck the lift pump flow.
    • If no restriction exists, replace the lift pump and recheck the lift pump flow. Refer to Fuel Lift Pump Replacement .
  6. Attach the lift pump suction line to the fuel sender.

Fuel Lift Pump Pressure Check
  1. Install a tee adapter at the injection pump.
  2. Connect a pressure gauge with a dial indication of 0-103 kPa (0 to 15 psi) to the tee adapter.
  3. Start the engine.
  4. Measure the fuel pressure.
    5. • If the fuel pressure is a least 4 psi (27 kPa), continue to step 4.
    • If the pressure is less than 4 psi, refer to Fuel Pump Relay Circuit Diagnosis before replacing the lift pump.
  5. Remove the pressure gauge and the tee adapter.
  6. Install the inlet pipe.
  7. Clean any of the fuel spillage
  8. Operate the engine.
  9. Check for any fuel leaks.

Fuel System Air Leak Check
  1. Install a transparent hose between the filter outlet and the injection pump inlet.
  2. Start the engine.
  3. Allow the engine to idle at the normal operating temperature.
  4. Check the fuel for air bubbles.
    5. • If there are no air bubbles present, stop the engine and Go to Step 10.
    • If there are air bubbles present, stop the engine and Go to Step 5.
  5. Check the lift pump suction line for air leakage.
    6. • Disconnect the fuel pipe from the fuel sender.
    • Plug the fuel pipe.
    • Disconnect the fuel pipe from the lift pump.
    • Install a hand held vacuum pump with a gauge to the fuel pipe.
    • Apply vacuum to the fuel pipe.
    • Observe the gauge reading.
       - If the vacuum does not drop, connect the fuel pipe and Go to Step 6.
       - If the vacuum drops, repair the air leak in the suction line and install the suction line pipe and the hose.
  6. Check the fuel sender for air leakage.
    7. • Remove the fuel tank.
    • Remove the fuel sender from the fuel tank.
    • Remove the strainer from the fuel sender.
    • Plug the bottom end of the pickup tube.
    • Apply a vacuum to the upper end of the pickup tube.
    • Observe the gauge reading.
       - If the vacuum does not drop (if the fuel inlet side of the sender is OK), install the fuel sender and the fuel tank.
       - If the vacuum drops, replace the fuel sender, install the fuel tank, connect the fuel pipe and Go to Step 7.
  7. Start the engine.
  8. Allow the engine to idle at the normal operating temperature.
  9. Observe the fuel for air bubbles.
    10. • If any air bubbles are present, stop the engine and recheck Steps 5 and 6.
    • If air bubbles are not present, stop the engine and Go to Step 10.
  10. Remove the transparent hose.
  11. Connect the hose of the filter outlet to the injection pump inlet fitting.
  12. Disconnect the return hose at the injection pump.
  13. Connect a transparent hose between the injection pump and the hose of the return line.
  14. Start the engine.
  15. Allow the engine to run.
  16. Observe the fuel for air bubbles.

    17.    Important: Small amounts of bubbles during snap acceleration is normal.

    • If any air bubbles are present, replace the injection pump. Refer to Fuel Injection Pump Replacement.
    • If there are no air bubbles present, Go to Step 17.
  17. Stop the engine.
  18. Remove the transparent hose.
  19. Connect the fuel return hose at the injection pump.
  20. Clean any fuel spillage which may be present.
  21. Run the engine to check for any fuel leakage.

Fuel Lift Pump Electrical Circuit (EFI)

When the key is first turned ON without the engine running, the control module turns the fuel lift pump relay ON during the glow plug cycle. This builds up the fuel pressure quickly. If the engine does not start after the glow plug cycle, the control module shuts the fuel lift pump OFF and waits for the engine RPM. As soon as the engine is cranked, the control module turns the relay ON and runs the fuel lift pump.

As a back up system to the fuel lift pump relay, the fuel lift pump is also turned ON by an oil pressure switch. When the engine oil pressure reaches about 28 kPa (4 psi) through cranking, the oil pressure switch will close in order to complete the circuit to the fuel pump.

For the location of the fuel pump relay, refer to Component Locator. For diagnosis of the lift pump electrical circuit, refer to Fuel Pump Relay Circuit Diagnosis .

Fuel Lift Pump Electrical Circuit (MFI)

The electrical circuit for the lift pump involves several main components:

    • The oil pressure switch/sending unit that is threaded into a lubrication system passage.
    • A mounting relay.
    • The lift pump itself that is mounted under the vehicle on the frame rail.

When the ignition switch is in the crank position, the lift pump circuit is completed through the relay contacts. During this time, oil pressure is building to the point of closing the contacts of the oil pressure switch. A minimum of 28 kPa (4 psi) is required to close the oil pressure switch contacts.

When the ignition switch is returned to the RUN position, the oil pressure of the running engine maintains electrical power to the lift pump. For diagnosis of the lift pump electrical circuit, refer to Fuel Pump Relay Circuit Diagnosis .