GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC MD Steel Tilt (W Model) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel System Diagnosis

Circuit Description

When you turn ON the ignition switch, the powertrain control module (PCM) 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 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 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

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

    5.

  5. 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.

    6.

  6. 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.

    7.

  7. 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.

    8.

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

    9.

  9. 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.

Step

Action

Value(s)

Yes

No

Schematic Reference: Central SFI Fuel System Overview

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. Remove the engine cover. Refer to Engine Cover Replacement in Interior Trim.
  4. Install the fuel pressure gauge.
  5. Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
  6. Turn ON the ignition, with the engine OFF.
  7. Command the fuel pump ON with a scan tool.
  8. Bleed the air out of the fuel pressure gauge.

    9. Important: 

       • It 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.

  9. Command the fuel pump ON with a scan tool.
  10. Observe the fuel pressure gauge, with the fuel pump commanded ON.

Is the fuel pressure within the specified value?

414-455 kPa (60-66 psi)

Go to Step 4

Go to Step 10

4

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 gauge for 1 minute.

Does the fuel pressure drop more than the specified value?

34 kPa (5 psi)

Go to Step 8

Go to Step 5

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

Does the fuel drop more than the second specified value?

69 kPa (10 psi)

14 kPa (2 psi)

Go to Step 22

Go to Step 6

6

  1. Turn ON the ignition, with the engine OFF.
  2. Monitor the fuel pressure gauge.
  3. 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 7

Go to Step 21

7
  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 15

Go to Symptoms - Engine Controls

8

  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Disconnect the fuel filter and the fuel return union fitting. Refer to Fuel Filter Replacement .
  4. Drain any remaining fuel into an approved gasoline container.
  5. Install the fuel pipe shut off adapters J 42873-2 and J 42873-1 in place of the fuel filter and the union fitting.
  6. Open the valves on the fuel pipe shut off adapters.
  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 gauge.
  10. Command the fuel pump ON and then OFF with a scan tool.
  11. Close the fuel feed pipe shut off valve.
  12. Monitor the fuel pressure gauge for 1 minute.

Does the fuel pressure remain constant?

21-69 kPa (3-10 psi)

Go to Step 17

Go to Step 9

9

  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 gauge for 1 minute.

Does the fuel pressure remain constant?

--

Go to Step 21

Go to Step 18

10

Is the fuel pressure above the specified value?

455 kPa (66 psi)

Go to Step 12

Go to Step 11

11

Is the fuel pressure above the specified value?

0 kPa (0 psi)

Go to Step 14

Go to Step 15

12

  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Disconnect the rear fuel return pipe union fitting located near the fuel filter.
  4. Attach a length of flexible fuel hose to the fuel chassis pipe.
  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 gauge while the fuel pump is operating.

Is the fuel pressure within the specified value?

414-455 kPa (59-66 psi)

Go to Step 20

Go to Step 13

13

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

Did you find and correct the condition?

--

Go to Step 23

Go to Step 21

14

  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
  3. Disconnect the rear fuel return pipe union fitting located near the fuel filter.
  4. Install the fuel pipe shut off adapter J 42873-2 in place of the fuel return pipe union.
  5. Open the valve in the fuel pipe shut off adapter.
  6. Turn ON the ignition with the engine OFF.
  7. Bleed the air from the fuel system.
  8. Monitor the fuel pressure gauge.
  9. Slowly close the valve in the fuel return pipe shut off adapter.
  10. Command the fuel pump ON with a scan tool.

Does the fuel pressure rise above the specified value?

455 kPa (66 psi)

Go to Step 21

Go to Step 15

15

Inspect the following for a restriction:

    • Fuel filter
    • Fuel feed pipe

Did you find and correct the condition?

--

Go to Step 23

Go to Step 16

16

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

Did you find and correct the condition?

--

Go to Step 23

Go to Step 17

17

  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 23

Go to Step 22

18

  1. Open the fuel return shut off valve.
  2. Remove the fuel meter body. Refer to Fuel Meter Body Assembly Replacement .
  3. Install the engine compartment fuel feed and fuel return pipes to the fuel meter body.
  4. Turn ON the ignition, with the engine OFF.
  5. Inspect for a fuel leak from the fuel pressure regulator.

Is the fuel pressure regulator leaking fuel?

--

Go to Step 21

Go to Step 19

19

Remove the fuel injector poppet nozzles from the intake manifold in order to isolate the leaking fuel injector. Refer to Fuel Injector Replacement .

Did you complete the replacement?

--

Go to Step 23

--

20

Locate and repair the restricted fuel return line between the chassis fuel pipe and the fuel tank.

Is the repair complete?

--

Go to Step 23

--

21

Important: Inspect for a missing or damaged 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 23

--

22

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

Is the replacement complete?

--

Go to Step 23

--

23

Operate the system in order to verify the repair.

Did you correct the condition?

--

System OK

Go to Step 3