GM 6.5L V8 – Fuel System – Fuel Supply System Checks

1. Check the output volume of the lift pump in the following steps:
A. Remove the electrical connector for the fuel shut-off solenoid at the injection pump.
B. Remove the pipe at the lift pump outlet fitting.
C. Install a hose at the lift pump outlet fitting and place a 1-liter (0.946-quart) container at the hose to collect fuel.
D. Crank the engine and measure the fuel volume (see Figure 7-62):
• If the volume is more than 0.24 liter (1/2 pint) in 15 seconds, check lift pump output pressure (part 3).
• If the volume is less than 0.24 liter (1/2 pint) in 15 seconds, check the lift pump suction line (part 2).

2. Check the suction line of the lift pump in the following steps:
A. Remove the fuel tank cap and repeat the lift pump output volume check (step 1):
• If the volume is more than 0.24 liter (1/2 pint) in 15 seconds, replace the defective fuel tank cap and go to part 3.
• If the volume is less than 0.24 liter (1/2 pint) in 15 seconds, go to step B.

B. Separate the lift pump suction line from the fuel pick-up/sending unit.
C. Connect the suction line to a source of clean fuel, using an additional hose.
D. Repeat the lift pump output volume check (see Figure 7-63):
• If the volume is more than 0.24 liter (1/2 pint) in 15 seconds, remove the fuel pick-up/sending unit and check it for restriction.
• If the volume is less than 0.24 liter (1/2 pint) in 15 seconds, go to step E.

Note: If the fuel pick-up filter is restricted with waxed fuel, bring the fuel to operating temperature before performing further tests.

E. Check the lift pump suction line for restriction:
• If restriction exists, repair it and re-check lift pump output volume.
• If no restriction exists, replace the lift pump and re-check lift pump output volume.
F. Attach the lift pump suction line to the fuel pick-up/sending unit and check lift pump output pressure
(part 4).

3. Check the output pressure of the lift pump in the following steps:
A. Install a tee adaptor between the lift pump outlet fitting and pressure line.
B. Connect a pressure gauge with a dial indication of 0 to 103 kPa (0 to 15 psi) to the tee adaptor.
C. Run the engine and measure the fuel pressure (see Figure 7-64):
• If the pressure is between 40 and 60 kPa (5.8 and 8.7 psi), go to step D.
• If the pressure is less than 40 kPa (5.8 psi), replace the lift pump and re-check output pressure.
D. Remove the pressure gauge and tee adaptor.
E. Install the pressure line at the lift pump outlet fitting.
F. Clean any fuel spillage.
G. Run the engine to check for fuel leakage
H. Check the lift pump pressure line (part 4).

4. Check the lift pump pressure line in the following steps:
A. Separate the hose of the lift pump pressure line from the fuel filter inlet fitting.
B. Install an extension hose at the lift pump pressure line and place a 1-liter (0.946-quart) container at the hose to collect fuel.
C. Repeat the lift pump output volume check (see Figure 7-65):
• If the volume is more than 0.24 liter (1/2 pint) in 15 seconds, check the fuel filter.
• If the volume is less than 0.24 liter (1/2 pint) in 15 seconds, go to step D.
D. Check the lift pump pressure line for restriction:
• If restriction exists, repair it and re-check lift pump output volume.
• If no restriction exists, replace the lift pump and re-check lift pump output volume.
E. Attach the hose of the lift pump pressure line to the fuel filter.
F. Clean any fuel spillage.
G. Run the engine to check for fuel leakage.
H. Check the fuel filter (part 5).

5. Check the fuel filter in the following steps:
A. Remove the electrical connector for the fuel shut-off solenoid at the injection pump.
B. Separate the hose of the filter outlet to the injection pump inlet fitting.
C. Install an extension hose at the filter outlet and place a 1-liter (0.946-quart) container at the hose to
collect fuel.
D. Repeat the lift pump output volume check (see Figure 7-66):
• If the volume is more than 0.24 liter (1/2 pint) in 15 seconds, go to step E.
• If the volume is less than 0.24 liter (1/2 pint) in 15 seconds, replace the fuel filter element and re-check lift pump output volume.

Note: If the fuel filter element is restricted with waxed fuel, bring the fuel to operating temperature before performing further tests. Also check the operation of the fuel heater (see page 7-21).

E. Install a tee adaptor between the lift pump outlet fitting and pressure line.

F. Connect a pressure gauge with a dial indication of 0 to 103 kPa (0 to 15 psi) to the tee adaptor.
G. Install the electrical connector for the fuel shut-off solenoid at the injection pump, then start and run
the engine.
H. Repeat the lift pump output pressure check (see Figure 7-67):
• If the pressure is between 40 and 60 kPa (5.8 and 8.7 psi), go to step I.
• If the pressure is less than 40 kPa (5.8 psi), replace the fuel filter element and re-check lift pump
output pressure.
I. Remove the pressure gauge and tee adaptor.
J. Check the fuel supply for air leaks (part 6).

6. Check the fuel supply for air leaks, following these steps:
A. Install a transparent hose between the filter outlet and injection pump inlet.
B. Start and run the engine, observing the fuel for air bubbles (see Figure 7-68):
• If air bubbles are not present, stop the engine and go to step F.
• If air bubbles are present, stop the engine and go to step C.
C. Check the lift pump suction line for air leakage:
• Separate the suction line hose from the fuel pick-up/sending unit and plug it.
• Separate the suction line pipe from the lift pump, and connect a vacuum pump/gauge to it.
• Apply a vacuum to the suction line, and observe the gauge reading:
– If vacuum does not drop, attach the suction line pipe and go to step D.
– If vacuum drops, repair the air leak in the suction line and attach the suction line pipe and hose.

D. Check the fuel pick-up/sending unit for air leakage:
• Remove the fuel tank.
• Remove the fuel pick-up/sending unit from the fuel tank, remove the filter and plug the bottom end of
the pick-up tube.
• Apply a vacuum to the upper end of the pick-up tube, and observe the gauge reading (see Figure
7-69):
– If vacuum does not drop, install the fuel pick-up sending unit and fuel tank.
– If vacuum drops, replace the fuel pick-up/sending unit, install the fuel tank, attach the suction line
hose and go to step E.
E. Start and run the engine, observing the fuel for air bubbles:
• If air bubbles are present, stop the engine and re-check steps C and D.
• If air bubbles are not present, stop the engine and go to step F.
F. Remove the transparent hose and attach the hose of the filter outlet to the injection pump inlet fitting.
G. Clean any fuel spillage.
H. Run the engine to check for fuel leakage.

 

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