GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Sonoma Pickup - 2WD 🢒 Body and Accessories 🢒 Wiring Systems 🢒 Fuel System Diagnosis

System Description

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

An electric fuel pump pumps the fuel through an in-line filter to the fuel meter body assembly. The pump attaches to the fuel 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. 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 for a loss of fuel pressure between the shut-off adapter on the fuel feed pipe and the fuel pump.

    4.

  4. 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.

    5.

  5. 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

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

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 3

Go to Fuel Pump Electrical Circuit Diagnosis

3

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. Install the fuel pressure gage. Refer to Fuel Pressure Gage Installation and Removal .
  4. Turn ON the ignition, with the engine OFF.

    5. Important: 

       • May be necessary to command the fuel pump ON a few times in order to obtain the highest possible fuel pressure.
       • DO NOT start the engine.

  5. Command the fuel pump ON with a scan tool.
  6. Observe the fuel pressure gage, with the fuel pump commanded ON.

Is the fuel pressure within the specified range?

384-425 kPa (56-62 psi)

Go to Step 4

Go to Step 9

4

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.

Monitor the fuel pressure gage for 1 minute.

Does the fuel pressure decrease by more than the specified value?

34 kPa (5 psi)

Go to Step 7

Go to Step 5

5
  1. Relieve the fuel pressure to the first specified value. Refer to Fuel Pressure Relief .
  2. Monitor the fuel pressure gage for 5 minutes.

Does the fuel pressure decrease by more than the second specified value?

69 kPa (10 psi)

14 kPa (2 psi)

Go to Step 17

Go to Step 6

6
  1. Operate, the vehicle within the conditions of the customer's 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 12

Go to Symptoms - Engine Controls

7

  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Disconnect the fuel feed hose from the fuel rail pipe. Refer to Metal Collar Quick Connect Fitting Service .
  4. Install the J 37287 Fuel Pipe Shut-off Adapter between the fuel hose and the fuel rail pipe.
  5. Open the valve on the fuel feed 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 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 8

Go to Step 15

8
  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Open the valve in the fuel feed pipe shut-off adapter.
  4. Disconnect the fuel return pipe from the fuel filter assembly.
  5. Install the J 37287 between the fuel return pipe and the fuel filter assembly.
  6. Open the valve in the fuel return pipe shut-off adapter.
  7. Turn ON the ignition, with the engine OFF.
  8. Command the fuel pump ON with a scan tool.
  9. Bleed the air from the fuel pressure gage.
  10. Command the fuel pump ON and then OFF with a scan tool.
  11. Close the fuel return pipe shut-off valve.
  12. Monitor the fuel pressure gage for 1 minute.

Does the fuel pressure remain constant?

--

Go to Step 16

Go to Step 17

9

Is the fuel pressure more than the specified value?

425 kPa (62 psi)

Go to Step 10

Go to Step 11

10

Inspect the fuel return pipe and for a restriction.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 16

11

  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Disconnect the fuel return pipe from the fuel filter assembly.
  4. Install the J 37287 between the fuel return pipe and the fuel filter assembly.
  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 pipe shut-off adapter.
  11. Command the fuel pump ON with a scan tool.

Does the fuel pressure increase to more than the specified value?

425 kPa (62 psi)

Go to Step 16

Go to Step 12

12

Inspect the following components for a restriction:

    • The fuel filter
    • The fuel feed pipe

Did you find and correct the condition?

--

Go to Step 18

Go to Step 13

13

Inspect the harness connectors and the 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 18

Go to Step 14

14
  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 18

Go to Step 17

15
  1. Turn OFF the ignition.
  2. Raise the fuel rail, with the fuel pipes 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 .

Did you complete the replacement?

--

Go to Step 18

--

16

Replace the fuel filter assembly. Refer to Fuel Filter Replacement .

Did you complete the replacement?

--

Go to Step 18

--

17

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

Did you complete the replacement?

--

Go to Step 18

--

18

Operate the system in order to verify the repair.

Did you correct the condition?

--

System OK

Go to Step 3