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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 271 (ENGINE CONTROLS - SYSTEM & COMPONENT TESTING - 3.8L)FUEL SYSTEMSFUEL TANK LEAK TESTDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedure

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Pontiac Grand Prix, 2003 Pontiac Bonneville, 2003 Chevrolet Monte Carlo, 2003 Chevrolet Impala, and 2003 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 3.8L BONNEVILLE, GRAND PRIX, IMPALA, LESABRE, MONTE CARLO, PARK AVENUE & REGAL article. After performing Diagnostic System Check - Engine Controls, go to next step.
  2. Raise and support the vehicle. Check the fuel tank and the fuel pipes for damage or for external leaks. Did you find fuel leaking from the fuel tank? If yes, go to step  7. If no, go to next step.
  3. Turn ignition switch to RUN position, engine OFF. Command the fuel pump ON using a scan tool. Check for fuel leaking from the fuel pipes. Did fuel leak from the fuel pipes? If yes, go to step  8. If no, go to next step.
  4. Install the Fuel Tank Cap Adapter (J 41415-40), or equivalent. Connect the Evaporative Emissions System Tester (J 41413-200). Seal the EVAP system using a scan tool. DO NOT exceed 15 in. H2O. Pressurize the fuel tank using the Evaporative Emissions System Tester. Did fuel leak from the fuel tank? If yes, go to step  7. If no, go to next step.
  5. Maintain pressure in the fuel tank using the Evaporative Emissions System Tester. The fuel tank may need to be partially lowered. Check for leaks above the fuel level in the following locations using the Ultrasonic Leak Detector (J 41416), or equivalent:
    • The fuel tank, the fill limiter vent valve, the pressure relief valve, and the rollover valves.
    • The fuel sender housing and fuel sender seal.
    • The Fuel Tank Pressure (FTP) sensor seal.
    • The EVAP vapor pipes.
    • The fuel fill pipe, the fuel fill hose, and the fuel fill cap.

    Did you locate a fuel or vapor leak? If yes, go to next step. If no, go to DIAGNOSTIC AIDS .

  6. Repair system as necessary.
  7. Replace fuel tank.
  8. Replace leaking fuel pipe.
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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.