GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Safari - 4WD 🢒 Body and Accessories 🢒 Wiring Systems 🢒 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

The number below refers to the step number 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 decreases during this test 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 decrease by 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 increases to more than 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 of the fuel pump electrical circuits thoroughly.

    9.

  9. This step checks 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

Values

Yes

No

Schematic Reference:

Central SFI Fuel System with Fuel Pressure Gauge


Object Number: 13109 Size: SF
(1)Fuel Inlet
(2)Bleed Hose
(3)J 34730-1A Fuel Pressure Gage Assembly
(4)Fuel Pressure Connection
(5)In-Line Fuel Filter
(6)Fuel Line Pressure Side
(7)Fuel Pump Feed Hose
(8)In-Tank Fuel Pump
(9)Fuel Pump Strainer
(10)Return Line
(11)Flexible Hose
(12)Fuel Return Line

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, leaving the engine OFF.
  2. Use a scan tool in order to command the fuel pump ON.

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 diagnosis.

  1. Turn OFF the ignition.
  2. Turn OFF all accessories.
  3. Remove the engine cover. Refer to Engine Cover Replacement in Interior Trim.

    4. 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.

  4. Install the fuel pressure gage.
  5. Place the bleed hose of the fuel pressure gage into an approved gasoline container.
  6. Turn ON the ignition, leaving the engine OFF.
  7. Use the scan tool in order to command the fuel pump ON.
  8. Bleed the air out of the fuel pressure gage.

    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. Use the scan tool in order to command the fuel pump ON.
  10. Observe the fuel pressure gage, with the fuel pump commanded ON.

Is the fuel pressure within the specified range?

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 gage for 1 minute.

Does the fuel pressure decrease by 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 gage for 5 minutes.

Does the fuel decrease 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, leaving the engine OFF.
  2. Monitor the fuel pressure gage.
  3. Start the engine.

Does the fuel pressure decrease by 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 customer's concern.
  2. Use a scan tool in order to monitor the fuel related parameters.

Do any of the 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 rear fuel feed pipe and the rear fuel return pipe from the chassis fuel pipes. Refer to Plastic Collar Quick Connect Fitting Service .
  4. Drain any remaining fuel into an approved gasoline container.
  5. Install the fuel pipe shut-off adapters between the rear fuel pipes and the chassis fuel pipes.
  6. Open the valves on the fuel pipe shut-off adapters.
  7. Turn ON the ignition, leaving the engine OFF.
  8. Use the scan tool in order to command the fuel pump ON.
  9. Bleed the air from the fuel pressure gage.
  10. Use the scan tool in order to command the fuel pump ON and OFF.
  11. Close the fuel feed pipe shut-off valve.
  12. Monitor the fuel pressure gage 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, leaving the engine OFF.
  4. Use the scan tool in order to command the fuel pump ON and OFF.
  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 21

Go to Step 18

10

Is the fuel pressure more than the specified value?

455 kPa (66 psi)

Go to Step 12

Go to Step 11

11

Is the fuel pressure more than 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 from the chassis pipe. Refer to Plastic Collar Quick Connect Fitting Service .
  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, leaving the engine OFF.
  7. Monitor the fuel pressure gage while the fuel pump is operating.

Is the fuel pressure within the specified range?

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 from the chassis pipe. Refer to Plastic Collar Quick Connect Fitting Service .
  4. Install the fuel pipe shut-off adapter between the rear fuel return pipe and the chassis fuel pipe.
  5. Open the valve in the fuel pipe shut-off adapter.
  6. Turn ON the ignition, leaving the engine OFF.
  7. Bleed the air from the fuel system.
  8. Monitor the fuel pressure gage.

    9. Important: Do not allow the fuel pressure to exceed 517 kPa (75 psi). Excess pressure may damage the fuel system.

  9. Slowly close the valve in the fuel return pipe shut-off adapter.
  10. Use the scan tool in order to command the fuel pump ON.

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

455 kPa (66 psi)

Go to Step 21

Go to Step 15

15

Inspect the following components for a restriction:

    • The fuel filter
    • The 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 Conditions and Poor Connections and to 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 for the following conditions:
    3. • Any damage to the fuel pump flex hose
    • Any poor connections at the in-tank fuel pump connectors
    • A restriction of the fuel strainer
    • Any contaminants 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, leaving 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