1997 Isuzu Trooper LSFACTORY OEM SERVICE PROCEDURE

Fuel System Diagnosis

WARNING: Relieve fuel pressure before connecting fuel pressure gauge. See FUEL SYSTEM PRESSURE RELEASE  under FUEL SYSTEM.
  1. Perform On-Board Diagnostic (OBD) System Check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK  . After performing OBD system check, go to next step.
  2. Turn ignition off. Turn A/C system off. Relieve fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE under FUEL SYSTEM. Install Fuel Pressure Gauge (J-34730-1 ). Turn ignition on. Fuel pump should run for 2 seconds (it may be necessary to turn ignition on several times to obtain highest possible fuel pressure). If fuel pressure is 42-55 psi (3.0-3.9 kg/cm2 ), go to next step. If fuel pressure is not 42-55 psi (3.0-3.9 kg/cm2 ), go to step 17).
  3. After fuel pump has shut off, fuel pressure should 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 vehicle at normal operating temperature, turn ignition on. Allow fuel pressure to build (it may be necessary to turn ignition on several times to obtain highest possible fuel pressure). Start engine. If fuel pressure drops 3-15 psi (0.2-1.0 kg/cm2 ), go to next step. If fuel pressure does not drop 3-15 psi (0.2-1.0 kg/cm2 ), go to step 9).
  5. If fuel pressure is suspected of dropping off during acceleration, cruise or hard cornering, go to next step. If fuel pressure is okay, check for improper fuel.
  6. Inspect fuel filter and fuel supply line. If no restriction is found, go to next step. If restriction is found, repair as necessary.
  7. Remove fuel tank. Inspect fuel pump strainer for a restriction. Check fuel line for leaks. Verify correct fuel pump is installed. If no problem is found, go to next step. If a problem is found, repair as necessary.
  8. Replace fuel pump. Verify system is operating properly.
  9. Disconnect vacuum hose to fuel pressure regulator. With engine at idle, apply 12-14 in. Hg. of vacuum to fuel pressure regulator. If fuel pressure drops 3-15 psi (0.2-1.0 kg/cm2 ), go to next step. If fuel pressure does not drop 3-15 psi (0.2-1.0 kg/cm2 ), go to step 11).
  10. Locate and repair loss of vacuum to fuel pressure regulator. See VACUUM DIAGRAMS article. Verify system is operating properly.
  11. Replace fuel pressure regulator. Verify system is operating properly.
  12. Using scan tool or 10-amp fused jumper wire connected to fuel pump relay, pressurize fuel system. Using locking pliers, pinch fuel supply hose. If fuel pressure remains constant, go to next step. If fuel pressure does not remain constant, go to step 15).
  13. Check fuel supply line for leaks. Repair as necessary.
  14. Remove fuel tank. Check for leaks in hose or in-tank fuel line. Repair as necessary. If no leaks are found, go to step 8).
  15. Remove locking pliers from fuel supply hose. Using locking pliers, pinch fuel return line. Using scan tool or 10-amp fused jumper wire connected to fuel pump relay, pressurize fuel system. Pinch fuel supply hose. If fuel pressure does not remain constant, go to next step. If fuel pressure remains constant, go to step 11).
  16. Locate and replace leaking fuel injector(s). Verify system is operating properly.
  17. If fuel pressure is greater than 55 psi (3.9 kg/cm2 ), go to next step. If fuel pressure is not greater than 55 psi (3.9 kg/cm2 ), go to step 21).
  18. Relieve fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE  under FUEL SYSTEM. Disconnect fuel return line from fuel rail. Connect a flexible hose to fuel rail return outlet passage, and place open end of hose into approved gasoline container. Turn ignition off for about 10 seconds. Turn ignition on. While fuel pump is running, observe fuel gauge. If fuel pressure is 42-55 psi (3.0-3.9 kg/cm2 ), go to next step. If fuel pressure is not 42-55 psi (3.0-3.9 kg/cm2 ), go to step 20).
  19. Repair restriction in fuel return line. Verify system is operating properly.
  20. Check for restriction in fuel rail outlet passages. Repair as necessary. If there is no restriction, go to step 11).
  21. If fuel pressure is greater than 0 psi (0 kg/cm2 ), go to next step. If fuel pressure is not greater than 0 psi (0 kg/cm2 ), go to step 23).
    CAUTION: In the next step, DO NOT  allow fuel pressure to exceed 60 psi (4.2 kg/cm2 ), or fuel pressure regulator or fuel pressure gauge may be damaged.
  22. Using scan tool or 10-amp fused jumper wire connected to fuel pump relay, pressurize fuel system. Using pliers, gradually pinch fuel return hose. If fuel pressure is greater than 55 psi (3.9 kg/cm2 ), go to step 11). If fuel pressure is not greater than 55 psi (3.9 kg/cm2 ), go to step 7).
  23. Using scan tool or 10-amp fused jumper wire connected to fuel pump relay, command fuel pump ON. Remove gas cap, and listen for sound of fuel pump. If fuel pump is running, go to step 7). If fuel pump is not running, go to FUEL SYSTEM ELECTRICAL TEST  .
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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