GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Yukon XL - 4WD 🢒 Engine 🢒 Engine Controls - 4.8L, 5.3L, and 6.0L 🢒 Fuel System Diagnosis

System Description

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

The fuel tank stores the fuel supply. The electric fuel pump supplies fuel through an in-line fuel filter to the fuel injection system. The pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pressure regulator maintains the correct fuel pressure to the fuel injection system. A separate pipe returns unused fuel to the fuel tank.

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 the engine vacuum. With the engine vacuum applied, the pressure should drop to 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 for a leaking 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, there is a restriction in the fuel return pipe.

    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. Thoroughly inspect all fuel electrical circuits.

Fuel System Diagnosis

Step

Action

Values

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: Before you proceed with this diagnostic, inspect the fuel system for damage and for external leaks.

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

Does the fuel pump operate?

 

Go to Step 3

Go to Intermittent Conditions

3

Important: Before you proceed with this diagnostic, inspect the fuel system for damage and for external leaks.

  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 Gauge.
  4. Place the bleed house from 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 from the fuel pressure gage.

    8. Important: 

       • You may need 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, and observe the fuel pressure gage.

Is the fuel pressure within the specified value?

385-425 kPa

(55-62 psi) VIN: (V, T,U)

335-375 (48-54 psi) VIN: Z

Go to Step 4

Go to Step 12

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, then 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 9

Go to Step 5

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

Does the fuel pressure drop more than the second specified value?

69 kPa

(10 psi)

14 kPa

(2 psi)

Go to Step 24

Go to Step 6

6

  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 7

Go to Step 8

7
  1. Operate the vehicle within the conditions of the customer's concerns.
  2. Monitor the fuel-related parameters with a scan tool.

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

--

Go to Step 17

Go to Symptoms - Engine Controls

8
  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 21

Go to Step  23

9

  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 vacuum port of the fuel pressure regulator .

Is the fuel pressure regulator leaking fuel?

--

Go to Step 23

Go to Step 10

10

  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. Refer to Metal Collar Quick Connect Fitting Service .
  4. Install the J 37287 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, 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 19

Go to Step 11

11

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

Does the fuel pressure remain constant?

--

Go to Step 23

Go to Step 20

12

Is the fuel pressure more than the specified value?

427 kPa (62 psi) VIN: (V, T,U)

375 KPa (54 psi) VIN: Z

Go to Step 14

Go to Step 13

13

Is the fuel pressure more than the specified value?

0 kPa (0 psi)

Go to Step 16

Go to Step 17

14

  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 with the fuel pump operating.

Is the fuel pressure within the specified value?

385-425 kPa (55-62 psi) VIN: (V, T,U)

335-375 KPa (48-54 psi) VIN: Z

Go to Step 22

Go to Step 15

15

Inspect the fuel rail outlet passage for a restriction.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 23

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. Install the J 37287 between the fuel return pipe and the fuel rail.
  5. Open the valve on the fuel pipe shut-off adapter.
  6. Turn ON the ignition, with the engine OFF.
  7. Use a scan tool in order to command the fuel pump ON.
  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. Use a scan tool in order to command the fuel pump ON.

Does the fuel pressure rise above the specified value?

425 kPa (62 psi) VIN: (V, T,U)

375 KPa (54 psi) VIN: Z

Go to Step 23

Go to Step 17

17

Inspect the following components for restrictions:

    • The fuel filter
    • The fuel feed pipe

Did you find and correct the condition?

--

Go to Step 25

Go to Step 18

18

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

Did you find and correct the condition?

--

Go to Step 25

Go to Step 19

19
  1. Remove the fuel sender assembly. Refer to Fuel Sender Assembly Replacement .
  2. Inspect the following components:
    3. • The fuel pump flex hose for damage
    • The in-tank fuel pump harness connectors for bad connections
    • The fuel strainer for restrictions
    • The fuel tank for contaminants

Did you find and correct any conditions?

--

Go to Step 25

Go to Step 24

20
  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 the replace the leaking fuel injector Refer to Fuel Injector Replacement .

Did you complete the replacement?

--

Go to Step 25

--

21

Repair the vacuum source to the fuel pressure regulator.

Did you complete the repair?

--

Go to Step 25

--

22

Locate and repair the restricted fuel return line.

Did you complete the repair?

--

Go to Step 25

--

23

Important: Before your replace the fuel pressure regulator, inspect for a missing or damaged fuel pressure regulator O-ring seal.

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

Did you complete the replacement?

--

Go to Step 25

--

24

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

Did you complete the replacement?

--

Go to Step 25

--

25

Operate the system in order to verify the repair.

Did you correct the condition?

--

System OK

Go to Step 3