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Fuel System Diagnosis (3.1L & 3.4L)

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  1. If diagnostic system check has already been performed, go to next step. If diagnostic system check has not been performed, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS in appropriate SELF-DIAGNOSTICS article.
    2. NOTE: Inspect fuel system for damage or external leaks, before proceeding with this diagnostic.
  2. Turn ignition on, with engine off. Using scan tool, command fuel pump on. Does fuel pump operate? If yes, go to next step. If no, go to FUEL PUMP ELECTRICAL CIRCUIT DIAGNOSIS (3.1L CENTURY & GRAND PRIX; 3.4L IMPALA & MONTE CARLO)  or FUEL PUMP ELECTRICAL CIRCUIT DIAGNOSIS (3.1L MALIBU; 3.4L ALERO & GRAND AM)  .
  3. Ensure there is adequate fuel in fuel tank. Turn ignition off. Turn off all accessories. Install Fuel Pressure Gauge (J-34730-1A) and fuel pressure gauge fitting. Turn ignition on, with engine off. Command the fuel pump relay on with scan tool. Fuel pump will run for about 2 seconds. Command fuel pump relay on with scan tool in order to obtain the highest possible fuel pressure. DO NOT start engine. Observe fuel gauge, with fuel pump relay commanded on. Is fuel pressure within 52-59 psi (3.7-4.1 kg/cm 2 ). If yes, go to next step. If no, go to step  12.
    4. NOTE: Fuel pressure may vary slightly when fuel pump stops running. After fuel pump stops running, fuel pressure should stabilize and remain constant.
  4. Monitor fuel pressure gauge for one minute. Does fuel pressure decrease by more than 5 psi (0.4 kg/cm 2 ). If yes, go to step  9. If no, go to next step.
  5. Relieve fuel pressure to 10 psi (0.7 kg/cm 2 ). Monitor fuel pressure gauge for 5 minutes. Does fuel pressure decrease by more than 2 psi (0.1 kg/cm 2 )? If yes, go to step  24. If no, go to next step.
  6. Turn ignition off for 15 seconds. Turn ignition on, with engine off. Monitor the fuel pressure gauge. Start engine. Does the fuel pressure decrease by 3-10 psi (0.2-0.7 kg/cm 2 ) when engine is started? If yes, go to next step. If no, go to step  8.
  7. Operate vehicle within the conditions of the customer's concern. Using scan tool, monitor fuel trim and HO2S parameters. Do any of the scan tool parameters indicate a lean condition? If yes, go to step  17. If no, problem is intermittent.
  8. Disconnect vacuum hose from fuel pressure regulator. Monitor fuel pressure gauge. Operate engine at idle. Using a hand-held vacuum pump, apply 12-14 in. Hg (41-47 kPa) to fuel pressure regulator. Does fuel pressure decrease by 3-10 psi (0.2-0.7 kg/cm 2 ) when vacuum is applied? If yes, go to step  21. If no, go to step  23.
  9. Turn ignition off. Remove vacuum hose from fuel pressure regulator. Turn ignition on, with engine off. Command fuel pump on with scan tool. Inspect for fuel leak at fuel pressure regulator vacuum port. Is fuel pressure regulator leaking fuel? If yes, go to step  23. If no, go to next step.
  10. Turn ignition off. Relieve fuel pressure. Disconnect fuel feed hose and fuel return hose from fuel rail pipes. Install Fuel Pipe Line Shut-Off Adapters (J-37287) between the fuel hoses and fuel rail pipes. Open the valves on the adapters. Turn ignition on, with engine off. Using scan tool, command fuel pump relay on. Bleed air from fuel pressure gauge. Close fuel feed pipe shut-off valve. Command fuel pump relay on with a scan tool in order to obtain the highest possible fuel pressure. Monitor fuel pressure gauge for one minute. Does the fuel pressure remain constant? If yes, go to step  19. If no, go to next step.
  11. Turn ignition off. Open fuel feed pipe shut-off valve. Turn ignition on, with engine off. Command fuel pump relay on and then off with scan tool. Close fuel return pipe shut-off valve. Monitor fuel pressure gauge for one minute. Does the fuel pressure remain constant? If yes, go to step  23. If no, go to step  20.
  12. Is fuel pressure greater than 59 psi (4.1 kg/cm 2 )? If yes, go to step  14. If no, go to next step.
  13. Is fuel pressure greater than zero psi? If yes, go to step  16. If no, go to step  17.
  14. Turn ignition off. Relieve fuel pressure. Disconnect fuel return hose from fuel rail return pipe. Attach a length of flexible fuel hose to fuel rail outlet passage. Place the open end of fuel hose into an approved gasoline container. Turn ignition on, with engine off. Monitor fuel pressure gauge while fuel pump is operating. Is fuel pressure within 52-59 psi (3.7-4.1 kg/cm 2 )? If yes, go to step  22. If no, go to next step.
  15. Inspect fuel rail return pipe and fuel rail outlet passage for a restriction. Did you find and correct the condition? If yes, go to step  25. If no, go to step  23.
  16. Turn ignition off. Relieve fuel pressure. Disconnect fuel return hose from fuel rail return pipe. Install Fuel Pipe Shut-Off Adapters (J-37287) between fuel return pipe and fuel rail. Open shut-off adapter valve. Turn ignition on, with engine off. Bleed air from fuel system. Monitor fuel pressure gauge. DO NOT allow fuel pressure to exceed 75 psi (5.3 kg/cm 2 ). Excessive pressure may damage the fuel system. Slowly close valve in fuel return pipe shut-off adapter. Command fuel pump relay on with scan tool. Does the fuel pressure increase to greater than 52 psi (3.7 kg/cm2 ). If yes, go to step  23. If no, go to next step.
  17. Check fuel filter and fuel feed pipe for restriction. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  18. Inspect the harness connectors and ground circuits of the fuel pump for poor connections. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  19. Remove fuel sender assembly. Inspect fuel pump flex hose for damage, in-tank fuel pump harness connectors for poor connections, fuel strainer for a restriction or for contaminates in fuel tank. Did you find and correct the condition? If yes, go to step  25. If no, go to step  24.
  20. Turn ignition off. Raise fuel rail with fuel lines connected. Turn ignition on, with engine off. Command fuel pump relay on with scan tool. Locate and replace leaking fuel injector. After replacing injector, go to step  25.
  21. Repair vacuum source to fuel pressure regulator. After repairs, go to step  25.
  22. Locate and repair the restricted fuel return line. After repairs, go to step  25.
    23. NOTE: Inspect for a missing or damaged fuel pressure regulator "O" ring before replacing fuel pressure regulator.
  23. Replace fuel pressure regulator. After replacing fuel pressure regulator, go to step  25.
  24. Replace fuel pump. After replacing fuel pump, go to next step.
  25. Operate the system in order to verify the repair. Did you correct the condition? If yes, system is okay. If no, go to step  3.
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