GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 GMC K Pickup - 4WD 🢒 Body and Accessories 🢒 Wiring Systems 🢒 Fuel System Diagnosis

Circuit Description

When you turn ON the ignition, the vehicle control module (VCM) energizes the fuel pump relay which powers the fuel pump ON. The pump remains ON as long as the engine is cranking or running and the VCM receives reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM also turns ON the fuel pump for 2 seconds when you turn OFF the ignition.

An electric fuel pump pumps the fuel through an in-line filter to the fuel meter body assembly. The pump attaches to the fuel level sender assembly inside of the fuel tank. The fuel pump provides fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel returns to the fuel tank by the fuel return pipe.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. When the ignition is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gauge should read 414-455 kPa (60-66 psi). The spring pressure inside the fuel pressure regulator controls the fuel pressure.

    2.

  2. A fuel system that drops more than 34 kPa (5 psi) in 10 minutes has a leak in one or more of the following areas:

    3. • The fuel pump check valve
    • The fuel pump flex pipe
    • The valve or valve seat within the fuel pressure regulator
    • The fuel injectors
    • A fuel pipe

  3. A fuel system that drops more than 14 kPa (2 psi) in 10 minutes after being relieved to 69 kPa (10 psi) indicates a leaking fuel pump check valve.

    4.

  4. Fuel pressure that drops-off during acceleration, cruise, or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. You can diagnose a lean condition using a scan tool. If an extremely lean condition occurs, the heated oxygen sensors will stop toggling. The heated oxygen sensor output voltages will drop below 300 mV. The fuel injector pulse width will increase.

Important: Make sure the fuel system is not operating in the Fuel Cut-Off Mode. This can cause false indications by the scan tool.

  1. When you open the throttle quickly, the manifold pressure increases (low vacuum). The fuel pressure regulator increases the fuel pressure as the engine vacuum drops in order to compensate for engine load.

    2.

  2. A rich condition may result from the fuel pressure being above 455 kPa (66 psi). A rich condition may cause a DTC to set. Driveability conditions associated with rich conditions can include hard starting followed by black smoke and a strong sulfur smell in the exhaust.

    3.

  3. This test determines if the high fuel pressure is due to a restricted fuel return pipe or if the high fuel pressure is due to a faulty fuel pressure regulator.

    4.

  4. A lean condition may result from the fuel pressure being below 414 kPa (60 psi). A lean condition may cause a DTC to set. Driveability conditions associated with lean conditions can include hard starting, when the engine is cold, hesitation, poor driveability, lack of power, surging, and misfiring.

    5.

  5. Restricting the fuel return pipe with the J 42873-2 fuel pipe shut-off adapter causes the fuel pressure to rise above the regulated fuel pressure. Using a scan tool to pressurize the fuel system, the fuel pressure should rise above 455 kPa (66 psi) as the valve on the fuel pipe shut-off adapter connected to the fuel return pipe becomes partially closed.

    6.

  6. If the injectors are not grounded, check the spark plug associated with a particular fuel injector for fouling or saturation in order to determine if that particular fuel injector is leaking. If checking the spark plug associated with a particular fuel injector for fouling or saturation does not determine that a particular fuel injector is leaking, use the following procedure:

    7. 1. Remove the fuel meter body assembly.
    2. Reinstall the engine compartment fuel feed and return pipes to the fuel meter body and the chassis fuel pipes.
    3. Lift the fuel meter body assembly just enough to leave the fuel injector poppet nozzles in the ports.
    4. Pressurize the fuel system by using the scan tool fuel pump enable.
    5. Visually and physically inspect the fuel injector poppet nozzles for leaks.

Step

Action

Value(s)

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Turn the ignition OFF.
  2. Turn the air conditioning system OFF.
  3. Install the J 34730-1A fuel pressure gauge to the engine compartment fuel pressure test connection.
  4. Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
  5. Turn ON the ignition, leaving the engine OFF.
  6. Command the fuel pump ON using a scan tool.
  7. Bleed the air out of the fuel pressure gauge.
  8. Command the fuel pump ON using a scan tool.

    9. Important:: The fuel pump runs for approximately 2 seconds each time you command the fuel pump ON with the scan tool. Enable the fuel pump as necessary in order to achieve the highest possible fuel pressure.

  9. Monitor the fuel pressure with the fuel pump running.

Is the fuel pressure within the specified range?

414-455 kPa (60-66 psi)

Go to Step 3

Go to Step 11

3

Important:: The fuel pressure may vary slightly when the fuel pump stops running. After the fuel pump stops running, the fuel pressure should stabilize and remain constant.

Does the fuel pressure drop more than the specified value in 10 minutes?

34 kPa (5 psi)

Go to Step 8

Go to Step 4

4

Relieve the fuel pressure to the first specified value.

Does the fuel pressure drop more than the second specified value in 10 minutes?

69 kPa (10 psi) 14 kPa (2 psi)

Go to Step 20

Go to Step 5

5

Do you suspect the fuel pressure of dropping-off during acceleration, cruise, or hard cornering?

--

Go to Step 7

Go to Step 6

6

  1. Start the engine.
  2. Allow the engine to idle at normal operating temperature.
  3. Open the throttle quickly.
  4. Monitor the fuel pressure. The fuel pressure should approach the top of the specified range.

Does the fuel pressure rise quickly?

414-455 kPa (60-66 psi)

Go to Symptoms

Go to Step 7

7

Visually and physically inspect the following items:

    • The fuel filter for restriction
    • The fuel feed pipe for restriction
    • The fuel pump electrical wiring for high resistance

Did you find a problem?

--

Go to Step 24

Go to Step 16

8
  1. Relieve the fuel pressure. Refer to the Fuel Pressure Relief .
  2. Remove the fuel filter.
  3. Drain any remaining fuel from the fuel pipes into an approved gasoline container.
  4. Install the J 42873-1 fuel pipe shut-off adapter in the place of the fuel filter.
  5. Open the valve on the fuel pipe shut-off adapter.
  6. Reconnect the negative battery cable.
  7. Turn ON the ignition, leaving the engine OFF.
  8. Command the fuel pump ON using a scan tool.
  9. Inspect for any leaks at the fuel pipe shut-off adapter fittings.
  10. Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
  11. Bleed the air out of the fuel pressure gauge.
  12. Command the fuel pump ON using a scan tool.
  13. Wait for the fuel pressure to build.
  14. Close the valve in the fuel pipe shut-off adapter that is connected to the fuel feed pipe.

Does the fuel pressure remain constant?

--

Go to Step 20

Go to Step 9

9

Inspect the fuel feed pipe between the shut-off adapter and the engine fuel pipe for a leak.

Did you find a fuel leak?

--

Go to Step 24

Go to Step 10

10
  1. Open the valve in the fuel pipe shut-off adapter that is connected to the fuel feed pipe.
  2. Relieve the fuel pressure. Refer to the Fuel Pressure Relief .
  3. Disconnect the fuel return pipe union fitting located near the fuel filter.
  4. Drain any remaining fuel from the fuel pipes into an approved gasoline container.
  5. Install the J 42873-2 fuel pipe shut-off adapter in the place of the fuel return pipe union.
  6. Open the valve on the fuel return pipe shut-off adapter.
  7. Reconnect the negative battery cable.
  8. Turn ON the ignition, leaving the engine OFF.
  9. Command the fuel pump ON using a scan tool.
  10. Inspect for any leaks at the fuel pipe shut-off adapter fittings.
  11. Wait for the fuel pressure to build.
  12. Close the valve in the fuel pipe shut-off adapter that is connected to the fuel return pipe.

Does the fuel pressure remain constant?

--

Go to Step 21

Go to Step 22

11

Is the fuel pressure above the specified limit?

455 kPa (66 psi)

Go to Step 12

Go to Step 14

12

  1. Relieve the fuel pressure. Refer to the Fuel Pressure Relief .
  2. Disconnect the fuel return pipe union fitting located near the fuel filter.
  3. Attach a length of flexible fuel hose to the chassis fuel return pipe.
  4. Place the open end of the flexible fuel hose into an approved gasoline container.
  5. Reconnect the negative battery cable.
  6. Command the fuel pump ON using a scan tool.
  7. Observe the fuel pressure with the fuel pump running.

Is the fuel pressure within the specified range?

414-455 kPa (60-66 psi)

Go to Step 23

Go to step 13

13

Visually and physically inspect the following for a restriction:

    • The engine compartment fuel return pipe
    • The chassis fuel return pipe

Did you find a restriction?

--

Go to Step 24

Go to Step 21

14

Is the fuel pressure above the specified value?

0 kPa (0 psi)

Go to Step 15

Go to Step 17

15

  1. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  2. Disconnect the fuel return pipe union fitting located near the fuel filter.
  3. Drain any remaining fuel from the fuel pipes into an approved gasoline container.
  4. Install the J 42873-2 fuel pipe shut-off adapter in the place of the fuel return pipe union.
  5. Open the valve on the fuel return pipe shut-off adapter.
  6. Reconnect the negative battery cable.
  7. Turn ON the ignition, leaving the engine OFF.
  8. Command the fuel pump ON using a scan tool.
  9. Inspect for any leaks at the fuel pipe shut-off adapter fittings.
  10. Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
  11. Bleed the air out of the fuel pressure gauge.
  12. Command the fuel pump ON using a scan tool.
  13. Wait for the fuel pressure to build.

    14. Notice: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system.

  14. Slowly close the valve in the fuel pipe shut-off adapter that is connected to the fuel return pipe.

Does the fuel pressure rise above the specified value?

455 kPa (66 psi)

Go to Step 21

Go to Step 16

16

Inspect for the following conditions:

    • Inspect for low fuel pump voltage due to high resistance connections.
    • Inspect for a poor fuel pump ground.
    • Inspect the fuel strainer for a restriction.
    • Inspect the fuel pump flex pipe for leaks
        .
    • Verify the fuel pump is the correct fuel pump for this vehicle.

Did you find a problem in any of these areas?

--

Go to Step 24

Go to Step 20

17
  1. Turn ON the ignition, leaving the engine OFF.
  2. Command the fuel pump ON using a scan tool.
  3. Listen for the fuel pump running.

Does the fuel pump run?

--

Go to Step 18

Go to Fuel Pump Electrical Circuit Diagnosis

18

Visually and physically inspect for the following conditions:

    • No fuel in the fuel tank -- Do not rely on the fuel gauge reading.
    • The fuel filter for obstructions
    • The fuel feed pipe for a restriction
    • The fuel strainer for obstructions
    • The fuel pump flex pipe for leaks

Did you find a problem in any of these areas?

--

Go to Step 24

Go to Step 19

19

Inspect the fuel feed pipe between the shut-off adapter and the fuel sender for a leak.

Did you find a fuel leak?

--

Go to Step 24

Go to Step 20

20

Replace the fuel pump. Refer to Fuel Tank Fuel Pump Module Replacement .

Is the action complete?

--

System OK

--

21

Replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement .

Is the action complete?

--

System OK

--

22

Locate and replace any leaking or grounded fuel injectors. For grounded injector diagnosis, refer to Fuel Injector Circuit Diagnosis .

Is the action complete?

--

System OK

--

23

Locate and correct the restriction in the fuel return pipe.

Is the action complete?

--

System OK

--

24

Repair the problem as necessary.

Is the action complete?

--

System OK

--