GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 TopKick C-Series (Conventional) C4/C5 🢒 Engine 🢒 Engine Controls - 8.1L 🢒 Fuel System Diagnosis

System Description

When the ignition is turned ON, the powertrain control module (PCM) will turn ON the in-tank fuel pump. The in-tank fuel pump will remain ON as long as the engine is cranking or running and the PCM is receiving ignition reference pulses. If there are no ignition reference pulses, the PCM will turn the in-tank fuel pump OFF 2 seconds after the ignition is turned  ON or 2 seconds after the engine stops running.

The in-tank fuel pump is an electric pump attached to the fuel sender assembly. The fuel pump is designed to provide fuel at a pressure above the pressure needed by the fuel injectors. A fuel pressure regulator, attached to the fuel rail, keeps the fuel available to the fuel injectors at a regulated pressure. Unused fuel is returned to the fuel tank by a separate fuel return pipe.

Test Description

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

  1. This step verifies that the fuel pump is operating.

    2.

  2. This step tests for an internal fuel leak. If the fuel pressure drops during this test, then an internal loss of pressure is indicated.

    3.

  3. This step tests the fuel pressure regulator. The fuel pressure regulator is controlled by engine vacuum. With engine vacuum applied, the pressure should drop the specified value.

    4.

  4. This step inspects the fuel pressure regulator for a fuel leak from the vacuum port. If fuel leaks from the fuel pressure regulator while the fuel pump is commanded ON, replace the fuel pressure regulator.

    5.

  5. This step tests for a loss of fuel pressure between the fuel feed pipe shut off adapter and the fuel pump.

    6.

  6. This step tests for a leaking fuel injector, or fuel pressure regulator. If the fuel pressure remains constant during this test, the fuel injectors are not leaking fuel.

    7.

  7. This step tests for a restricted fuel return pipe. If the fuel pressure is within the specified values, a restriction in the fuel return pipe is indicated.

    8.

  8. This step determines if the fuel pressure regulator, or the fuel pump, is the cause of the low fuel pressure. If the pressure rises above the specified value, the fuel pump is OK.

    9.

  9. This step verifies that a circuit condition is not the cause of a fuel pressure concern. Inspect all fuel pump electrical circuits thoroughly.

Step

Action

Value(s)

Yes

No

Schematic Reference: Fuel System Description

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Is the vehicle equipped with dual fuel tanks?

--

Go to Step 3

Go to Step 4

3
  1. Turn ON the ignition, with the engine OFF.
  2. Observe the fuel level sensor parameter and the fuel level sensor rear parameter with a scan tool.

Is the fuel level sensor parameter less than the specified value and the fuel level sensor rear parameter more than the specified value?

1 V

Go to Secondary Fuel System Diagnosis

Go to Step 4

4

Important: Inspect the fuel system for damage, or external leaks, before proceeding with this diagnostic.

  1. Turn ON the ignition, with the engine OFF.
  2. Command the fuel pump ON with a scan tool.

Does the fuel pump operate?

--

Go to Step 5

Go to Fuel Pump Electrical Circuit Diagnosis

5

Important: Verify there is adequate fuel in the fuel tank before proceeding with this diagnostic.

  1. Turn OFF the ignition.
  2. Turn OFF all accessories.

    3. Caution: Wrap a shop towel around the fuel pressure connection in order to reduce the risk of fire and personal injury. The towel will absorb any fuel leakage that occurs during the connection of the fuel pressure gage. Place the towel in an approved container when the connection of the fuel pressure gage is complete.

  3. Install the J 34730-1A fuel pressure gage.
  4. Place the bleed hose of the fuel pressure gage into an approved gasoline container.
  5. Turn ON the ignition, with the engine OFF.
  6. Command the fuel pump ON with a scan tool.
  7. Bleed the air out of the fuel pressure gage.

    8. Important: 

       • It may be necessary to command the fuel pump ON several times, in order to obtain the highest possible fuel pressure.
       • Do not start the engine.

  8. Command the fuel pump ON with a scan tool.

    9. Observe the fuel pressure gage, with the fuel pump commanded ON.

Is the fuel pressure within the specified value?

384-425 kPa (55-62 psi)

Go to Step 6

Go to Step 14

6

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

Monitor the fuel pressure gage for 1 minute.

Does the fuel pressure drop more than the specified value?

34 kPa (5 psi)

Go to Step 11

Go to Step 7

7
  1. Relieve the fuel pressure to the first specified value.
  2. Monitor the fuel pressure gage for 5 minutes.

Does the fuel drop more than the second specified value?

69 kPa (10 psi)

14 kPa (2 psi)

Go to Step 26

Go to Step 8

8

  1. Turn OFF the ignition for 15 seconds.
  2. Turn ON the ignition, with the engine OFF.
  3. Monitor the fuel pressure gage.
  4. Start the engine.

Does the fuel pressure drop the specified value when the engine is started?

21-69 kPa (3-10 psi)

Go to Step 9

Go to Step 10

9
  1. Operate the vehicle within the conditions of the customers concern.
  2. Monitor the fuel related parameters with a scan tool.

Do any of the scan tool parameters indicate a lean condition?

--

Go to Step 19

Go to Symptoms - Engine Controls

10
  1. Disconnect the vacuum hose from the fuel pressure regulator.
  2. Monitor the fuel pressure gage.
  3. Idle the engine.
  4. Apply 12-14 inches of vacuum to the fuel pressure regulator with a hand held vacuum pump.

Does the fuel pressure drop the specified value when the vacuum is applied?

21-69 kPa (3-10 psi)

Go to Step 23

Go to Step 25

11

  1. Turn OFF the ignition.
  2. Remove the vacuum hose from the fuel pressure regulator.
  3. Turn ON the ignition, with the engine OFF.
  4. Command the fuel pump ON with a scan tool.
  5. Inspect for a fuel leak from the fuel pressure regulator vacuum port.

Is the fuel pressure regulator leaking fuel?

--

Go to Step 25

Go to Step 12

12

  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Disconnect the fuel feed hose and the fuel return hose from the fuel rail pipes. Refer to Metal Collar Quick Connect Fitting Service .
  4. Install the J 37287 fuel pipe shut off adapters between the fuel hoses and the fuel rail pipes.
  5. Open the valves on the fuel pipe shut off adapters.
  6. Turn ON the ignition, with the engine OFF.
  7. Command the fuel pump ON with a scan tool.
  8. Bleed the air from the fuel pressure gage.
  9. Command the fuel pump ON and then OFF with a scan tool.
  10. Close the fuel feed pipe shut off valve.
  11. Monitor the fuel pressure gage for 1 minute.

Does the fuel pressure remain constant?

--

Go to Step 21

Go to Step 13

13

  1. Turn OFF the ignition.
  2. Open the fuel feed pipe shut off valve.
  3. Turn ON the ignition, with the engine OFF.
  4. Command the fuel pump ON and then OFF with a scan tool.
  5. Close the fuel return pipe shut off valve.
  6. Monitor the fuel pressure gage for 1 minute.

Does the fuel pressure remain constant?

--

Go to Step 25

Go to Step 22

14

Is the fuel pressure above the specified value?

427 kPa (62 psi)

Go to Step 16

Go to Step 15

15

Is the fuel pressure above the specified value?

0 kPa (0 psi)

Go to Step 18

Go to Step 19

16

  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Disconnect the fuel return hose from the fuel rail return pipe. Refer to Metal Collar Quick Connect Fitting Service .
  4. Attach a length of flexible fuel hose to the fuel rail outlet passage.
  5. Place the open end of the flexible fuel hose into an approved gasoline container.
  6. Turn ON the ignition, with the engine OFF.
  7. Monitor the fuel pressure gage while the fuel pump is operating.

Is the fuel pressure within the specified value?

385-425 kPa (55-62 psi)

Go to Step 24

Go to Step 17

17

Inspect the fuel rail return pipe and fuel rail outlet passage for a restriction.

Did you find and correct the condition?

--

Go to Step 27

Go to Step 25

18

  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Disconnect the fuel return hose from the fuel rail return pipe. Refer to Metal Collar Quick Connect Fitting Service .
  4. Install the J 37287 between the fuel return pipe and the fuel rail.
  5. Open the valve in the fuel pipe shut off adapter.
  6. Turn ON the ignition, with the engine OFF.
  7. Command the fuel pump ON with a scan tool.
  8. Bleed the air from the fuel system.
  9. Monitor the fuel pressure gage.

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

  10. Slowly close the valve in the fuel return pipe shut off adapter.
  11. Command the fuel pump ON with a scan tool.

Does the fuel pressure rise above the specified value?

425 kPa (62 psi)

Go to Step 25

Go to Step 19

19

Inspect the following for a restriction:

    • The fuel filter
    • The fuel feed pipe

Did you find and correct the condition?

--

Go to Step 27

Go to Step 20

20

Inspect the harness connectors and ground circuits of the fuel pump for poor connections. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 27

Go to Step 21

21
  1. Remove the fuel sender assembly. Refer to Fuel Sender Assembly Replacement .
  2. Inspect the following items:
    3. • The fuel pump flex hose for damage
    • The in tank fuel pump harness connectors for poor connections
    • The fuel strainer for a restriction
    • Contaminates in the fuel tank

Did you find and correct the condition?

--

Go to Step 27

Go to Step 26

22
  1. Turn OFF the ignition.
  2. Raise the fuel rail, with the fuel lines connected. Refer to Fuel Injection Fuel Rail Assembly Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Command the fuel pump ON with a scan tool.
  5. Locate and replace the leaking fuel injector. Refer to Fuel Injector Replacement .

Is the replacement complete?

--

Go to Step 27

--

23

Repair the vacuum source to the fuel pressure regulator.

Is the repair complete?

--

Go to Step 27

--

24

Locate and repair the restricted fuel return line.

Is the repair complete?

--

Go to Step 27

--

25

Important: Inspect for a missing, or damaged, fuel pressure regulator O-ring seal before replacing the fuel pressure regulator.

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

Is the replacement complete?

--

Go to Step 27

--

26

Replace the fuel pump. Refer to Fuel Sender Assembly Replacement .

Is the replacement complete?

--

Go to Step 27

--

27

Operate the system in order to verify the repair.

Did you correct the condition?

--

System OK

Go to Step 5