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Home >> Buick >> 2004 >> Century Base >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls 3.1L (Troubleshooting & Diagnosis) >> Engine Controls >> Fuel System Diagnosis >> Test Description
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Test Description

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

  1. 2:  This step verifies that the fuel pump is operating.
  2. 4:  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. 6:  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. 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.
  5. 10:  This step tests for a loss of fuel pressure between the fuel feed pipe shut-off adapter and the fuel pump.
  6. 11:  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. 14:  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. 16:  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.
Fuel System Diagnosis

Step Action Values 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 
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 adequate fuel in the fuel tank before proceeding with this diagnostic.
  1. Turn OFF the ignition.
  2. Turn OFF all accessories.
  3. Install a J 34730-1A  Fuel Pressure Gage and the fuel pressure gage fitting. Refer to Fuel Pressure Gage Installation and Removal .
  4. Turn ON the ignition, with the engine OFF.
  5. Command the fuel pump relay ON with a scan tool.
    6. IMPORTANT:
    • The fuel pump runs for approximately 2 seconds. Command the fuel pump relay ON with a scan tool in order to obtain the highest possible fuel pressure.
    • Do not start the engine.
  6. Observe the J 34730-1A  , with the fuel pump relay commanded ON.
Is the fuel pressure within the specified value?		 358-405 kPa (52-59 psi) Go to Step 4  Go to Step 12 
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 J 34730-1A  for 1 minute.Does the fuel pressure decrease by 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 J 34730-1A  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 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 J 34730-1A  .
  4. 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 8 
7
  1. Operate the vehicle within the conditions of the customers concern.
  2. Monitor the fuel trim and HO2S parameters with a scan tool.
Do any of the scan tool parameters indicate a lean condition?		 - Go to Step 17  Go to Diagnostic Aids
8
  1. Disconnect the vacuum hose from the fuel pressure regulator.
  2. Monitor the J 34730-1A  .
  3. Idle the engine.
  4. Apply 34-47 kPa (12-14 in) of vacuum to the fuel pressure regulator with a J 23738-A  Vacuum Pump.
Does the fuel pressure decrease by 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 fuel pressure regulator vacuum port.
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 Procedure .
  3. Disconnect the fuel feed hose and the fuel return hose from the fuel rail pipes. Refer to Quick Connect Fitting(s) Service (Metal Collar) .
  4. Install the J 37287  Fuel Pipe Line Shut-Off Adapters between the fuel hoses and the fuel rail pipes.
  5. Open the valves on the J 37287  .
  6. Turn ON the ignition, with the engine OFF.
  7. Command the fuel pump relay ON with a scan tool.
  8. Bleed the air from the J 34730-1A  .
  9. Close the fuel feed pipe shut-off valve.
    10. IMPORTANT: Command the fuel pump relay ON with a scan tool in order to obtain the highest possible fuel pressure.
  10. Monitor the J 34730-1A  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 fuel feed pipe shut-off valve.
  3. Turn ON the ignition, with the engine OFF.
  4. Command the fuel pump relay ON and then OFF with a scan tool.
  5. Close the fuel return pipe shut-off valve.
  6. Monitor the J 34730-1A  for 1 minute.
Does the fuel pressure remain constant?		 - Go to Step 23  Go to Step 20 
12 Is the fuel pressure above the specified value? 407 kPa (59 psi) Go to Step 14  Go to Step 13 
13 Is the fuel pressure above 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 Procedure .
  3. Disconnect the fuel return hose from the fuel rail return pipe. Refer to Quick Connect Fitting(s) Service (Metal Collar) .
  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 J 34730-1A  while the fuel pump is operating.
Is the fuel pressure within the specified value?		 358-407 kPa (52-59 psi) Go to Step 22  Go to Step 15 
15 Inspect the fuel rail return pipe and 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 Procedure .
  3. Disconnect the fuel return hose from the fuel rail return pipe. Refer to Quick Connect Fitting(s) Service (Metal Collar) .
  4. Install the J 37287  between the fuel return pipe and the fuel rail.
  5. Open the valve in theJ 37287  .
  6. Turn ON the ignition, with the engine OFF.
  7. Bleed the air from the fuel system.
  8. Monitor the J 34730-1A  .
    9. NOTE: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system.
  9. Slowly close the valve in the fuel return pipe shut-off adapter.
  10. Command the fuel pump relay ON with a scan tool.
Does the fuel pressure rise above the specified value?		 358 kPa (52 psi) Go to Step 23  Go to Step 17 
17 Inspect the following for a restriction:
  • 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 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 25  Go to Step 19 
19
  1. Remove the fuel sender assembly. Refer to Fuel Sender Assembly Replacement .
  2. Inspect the following items:
    • 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 25  Go to Step 24 
20
  1. Turn OFF the ignition.
  2. Raise the fuel rail, with the fuel lines connected. Refer to Fuel Rail Assembly Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Command the fuel pump relay 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 25  -
21 Repair the vacuum source to the fuel pressure regulator.
Is the repair complete?		 - Go to Step 25  -
22 Locate and repair the restricted fuel return line.
Is the repair complete?		 - Go to Step 25  -
23
IMPORTANT: Inspect for a missing or damaged fuel pressure regulator O-ring before replacing the fuel pressure regulator.
Replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement .Is the replacement complete?		 - Go to Step 25  -
24 Replace the fuel pump. Refer to Fuel Sender Assembly Replacement .
Is the replacement complete?		 - 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 
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