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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 279 (ENGINE CONTROLS - SYSTEM & COMPONENT TESTING)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 is about a different car, the 2003 Cadillac Seville and 2003 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: Evaporative Emissions System Tester (J-41413-200), Fuel Tank Cap adapter (J-41415-40) and Ultrasonic Leak Detector (J-41416) are required for this procedure. See Figure -Figure .
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.6L DEVILLE & SEVILLE article.
    2. NOTE: Before performing this procedure, place a dry chemical (class B) fire extinguisher near the work area.
  2. Raise the vehicle. Inspect the fuel tank and fuel pipes for damage or external leaks. Did you find fuel leaking from the fuel tank? If yes, go to step  6 . If no, go to next step.
  3. Turn ON the ignition, with the engine OFF. Using a scan tool, command the fuel pump ON. Inspect for fuel leaking from the fuel pipes. Did fuel leak from the fuel pipes? If yes, go to step  7 . If no, go to next step.
  4. Install the Fuel Tank Cap Adapter. Connect the Evaporative Emissions System Tester (EEST) to the fuel tank cap adapter. Using a scan tool, seal the EVAP system. DO NOT exceed 15 in. H2O. Using the EEST, pressurize the fuel tank. Did fuel leak from the fuel tank? If yes, go to step  6 . If no, go to next step.
    5. NOTE: The fuel tank may need to be partially lowered.
  5. Maintain pressure in the fuel tank using the EEST. Using the Ultrasonic Leak Detector, test for leaks above the fuel level in the following locations.
    • The fuel tank, fill limiter vent valve, pressure relief valve and 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 and hose.

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

  6. Replace the fuel tank. See FUEL TANK under FUEL SYSTEMS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. After repair is completed, go to step  8
  7. Replace the leaking fuel pipe. Repair as necessary. After repair is completed, go to next step.
  8. Operate the vehicle in order to verify the repair. Did you correct the condition? If yes, system is okay. If no, go to DIAGNOSTIC AIDS  .
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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.