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Fuel System Diagnosis

  1. Perform On-Board Diagnostic II (OBD-II) System Check. See ON-BOARD DIAGNOSTIC II (OBD-II) SYSTEM CHECK in appropriate SELF-DIAGNOSTICS article. After performing OBD-II system check, go to next step.
  2. Turn ignition off. Turn A/C off. Relieve fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE  . Connect fuel pressure gauge. See Fig 1 . Turn ignition on (fuel pump will run for about two seconds). Use scan tool to command fuel pump ON. Note fuel pressure on gauge. If reading is not within specifications, go to step  17 . See FUEL PUMP PERFORMANCE  table under FUEL SYSTEM DIAGNOSIS. If reading is as specified, go to next step.
    Fig 1: Installing Fuel Pressure Gauge (Typical)
    G96G10104Courtesy of ISUZU MOTOR CO.
    FUEL PUMP PERFORMANCE

    Application Specification
    All Models
    Operating Pressure 42-55 psi (3.0-3.9 kg/cm2 )
    Pressure Drop 3-15 psi (0.2-1.1 kg/cm2 )
  3. Fuel pressure will drop when fuel pump stops running. It should then stabilize and remain constant. If fuel pressure remains constant, go to next step. If fuel pressure does not remain constant, go to step  12 .
  4. With engine warmed up to operating temperature, turn ignition on and observe fuel pressure. Continue to observe fuel pressure while starting engine. Reading should drop a specified amount when engine starts. See FUEL PUMP PERFORMANCE  table under FUEL SYSTEM DIAGNOSIS. If reading drops by the amount specified, go to next step. If reading does not drop by the amount specified, go to step  9 .
  5. Test drive vehicle to determine if fuel pressure is dropping off during acceleration, cruise or hard cornering. If it is, go to next step. If it is not, check for improper fuel in vehicle. Drain and replace as necessary.
  6. Check for restricted fuel filter or fuel line. Repair as necessary. If no problem was found, go to next step.
  7. Remove fuel tank to inspect for restricted fuel pump strainer, leaking fuel line or incorrect fuel pump. Repair as necessary. If no problem was found, go to next step.
  8. Replace fuel pump and test drive vehicle to confirm repair.
  9. Disconnect fuel pressure regulator vacuum hose. With engine idling, apply 12-14 in. Hg. while observing fuel pressure gauge. If fuel pressure drops specified amount, go to next step. See FUEL PUMP PERFORMANCE  table under FUEL SYSTEM DIAGNOSIS. If fuel pressure does not drop specified amount, go to step  11 .
  10. Inspect fuel pressure regulator vacuum hose for leaks. Repair or replace as necessary. Test drive vehicle to confirm repair.
  11. Replace fuel pressure regulator. Test drive vehicle to confirm repair.
  12. Use scan tool to command fuel pump ON. After pressure builds up, turn off pump and clamp fuel supply hose with locking pliers that will not damage fuel line. Observe fuel pressure. If pressure remains constant, go to next step. If fuel pressure does not remain constant, go to step  15 .
  13. Inspect fuel supply hose for leaks and repair as necessary. If no problems were found, go to next step.
  14. Remove fuel tank and inspect for leaking hose or in-tank fuel line. Repair as necessary. If no problem was found, go back to step  8 .
  15. Remove locking pliers from fuel supply hose, and clamp them to fuel return hose. Use scan tool to command fuel pump ON. After pressure has built up, turn off power and observe fuel pressure gauge. If fuel pressure reading remains constant, go back to step  11 . If fuel pressure reading does not remain constant, go to next step.
  16. Locate and replace any leaking fuel injectors. Test drive vehicle to confirm repair.
  17. If fuel pressure indicated on gauge is above 55 psi (3.9 kg/cm2 ), go to next step. If reading is not higher than specified, go to step  21 .
  18. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELEASE  table. Disconnect fuel return line from fuel rail. Attach flexible fuel hose to fuel rail return outlet and put other end into a suitable container. Use scan tool to command fuel pump ON. Observe fuel pressure while fuel pump in running. If fuel pressure is within specified limits, go to next step. See FUEL PUMP PERFORMANCE  table under FUEL SYSTEM DIAGNOSIS. If fuel pressure is not as specified, go to step  20 .
  19. Locate and repair restriction in fuel return line. Test drive vehicle and confirm repair.
  20. Visually inspect fuel rail outlet passages for restriction. Repair or replace as necessary. If no problem was found, go back to step  11 .
  21. If fuel pressure indicated is above 0 psi (0 kg/cm2 ), go to next step. If fuel pressure is 0 psi (0 kg/cm2 ), go to step  23 .
  22. Use scan tool to command fuel pump ON. Observe fuel pressure reading while slowly pinching off fuel return hose. If fuel pressure rises above 55 psi (3.9 kg/cm2 ), go back to step  11 . If fuel pressure does not rise above 55 psi (3.9 kg/cm2 ), go back to step  7 .
    CAUTION: DO NOT let fuel pressure rise above 60 psi (4.2 kg/cm2 ), or fuel pressure regulator damage may result.
  23. Use scan tool to command fuel pump ON. Remove fuel filler cap and listen for fuel pump operation. If fuel pump is running, go back to step  7 . If fuel pump is not running, go to FUEL SYSTEM ELECTRICAL TEST  .
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