GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1996 🢒 GMC Suburban - 4WD 🢒 Engine 🢒 Engine Controls - 6.5L 🢒 Fuel Supply System Check

If the fuel supply system is not delivering enough fuel, or air is being drawn into the fuel injection system, driveability could be greatly effected or a Cranks But Will Not Run symptom could exist. If other 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 pump output.
    • Restriction in the fuel return system.
    • Make certain that there is sufficient fuel in the tank.
    • Check for leaks at all fuel connections from the fuel tank to the injection pump.
    • Tighten any loose connections.
    • With the engine running, check all hoses and lines for flattening or kinks that would restrict the flow of fuel.

Fuel Lift Pump Flow Check
  1. Remove FUEL SOL fuse.
  2. Disconnect the pipe at the lift pump outlet fitting.
  3. Install a hose at the lift pump outlet fitting and place a 1 liter (0.946 quart) container at the hose to collect fuel.
  4. Crank the engine and measure the amount of fuel.
    5. • If more than 0.24 liter (½ pint) in 15 seconds, refer to Fuel Supply System Check.
    • If less than 0.24 liter (½ pint) in 15 seconds, refer to Fuel Supply System Check.

Fuel Lift Pump Suction Line Check
  1. Remove the fuel tank cap and repeat the Lift Pump Flow Check.
    2. • If flow is more than 0.24 liter (½ pint) in 15 seconds, replace the defective fuel tank cap and refer to Fuel Supply System Check
    • If flow is less than 0.24 liter (½ pint) in 15 seconds, go to next step.
  2. Separate the lift pump suction line from the fuel sender.
  3. Connect the suction line to a source of clean fuel, using an additional hose.
  4. Repeat the Lift Pump Flow Check.
    5. • If flow is more than 0.24 liter (½ pint) in 15 seconds, remove the fuel sender and check it for restriction.
    • If flow is less than 0.24 liter (½ pint) in 15 seconds, Go to Step 5.
  5. Check the lift pump suction line for restriction.
    6. • If restriction exists, repair it and recheck lift pump flow.
    • If no restriction exists, replace the lift pump and recheck 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 dial indication of 0-103 kPa (0 to 15 psi) to the tee adapter.
  3. Start engine and measure fuel pressure.
    4. • If fuel pressure is a least 4 psi (27 kPa) continue to step 4.
    • If pressure is less than 4 psi, refer to Fuel Pump Circuit Diagnosis before replacing lift pump.
  4. Remove pressure gauge and tee adapter.
  5. Connect inlet pipe.
  6. Clean any fuel spillage
  7. Operate engine and check for fuel leaks.

Fuel System Air Leak Check
  1. Install a transparent hose between the filter outlet and injection pump inlet.
  2. Start and idle the engine, observing the fuel for air bubbles.
    3. • If air bubbles are not present, stop the engine and Go to Step 6.
    • If air bubbles are present, stop the engine and Go to Step 3.
  3. Check the lift pump suction line for air leakage.
    4. • Disconnect the fuel pipe from the fuel sender and plug it.
    • Disconnect the fuel pipe from the lift pump, and install a hand held vacuum pump with gauge.
    • Apply vacuum to the fuel pipe and observe the gauge reading.
       - If vacuum does not drop, connect fuel pipe and Go to Step 4.
       - If vacuum drops, repair the air leak in the suction line and install the suction line pipe and hose.
  4. Check the fuel sender for air leakage.
    5. • Remove the fuel tank.
    • Remove the fuel sender from the fuel tank, remove strainer and plug the bottom end of the pickup tube.
    • Apply a vacuum to the upper end of the pickup tube, and observe the gauge reading.
       - If vacuum does not drop (fuel inlet side of sender is OK), install the fuel sender and fuel tank.
       - If vacuum drops, replace the fuel sender, install the fuel tank, connect the fuel pipe and Go to Step 5.
  5. Start and run the engine, observing the fuel for air bubbles.
    6. • If air bubbles are present, stop the engine and recheck Steps 3 and 4.
    • If air bubbles are not present, stop the engine and Go to Step 6.
  6. Remove the transparent hose and connect the hose of the filter outlet to the injection pump inlet fitting.
  7. Disconnect the return hose at the injection pump.
  8. Install a transparent hose between the injection pump and the hose of the return line.
  9. Start and run the engine, observing the fuel for air bubbles.

    10.    Important: Its OK to see small amounts of bubbles during snap acceleration.

    • If air bubbles are present, replace the injection pump. Refer to Fuel Injection Pump Replacement
    • If air bubbles are not present, Go to Step 10.
  10. Stop the engine.
  11. Remove the transparent hose and attach the fuel return hose at the injection pump.
  12. Clean any fuel spillage.
  13. Run the engine to check for fuel leakage.

Fuel Lift Pump Electrical Circuit

When the key is first turned ON without the engine running, the control module turns the fuel lift pump relay ON during glow plug cycle. This builds up fuel pressure quickly. If the engine is not started after the glow plug cycle, the control module shuts the fuel lift pump OFF and waits for 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 engine oil pressure reaches about 28 kPa (4 psi), through cranking the oil pressure switch will close to complete the circuit to the fuel pump.

For the location of the fuel pump relay, refer to On Vehicle Service. For diagnosis of the lift pump electrical circuit, refer to Fuel Pump Circuit Diagnosis