Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 153 (ENGINE CONTROLS - SYSTEM & COMPONENT TESTING)FUEL SYSTEMSFUEL TANK LEAK TESTDIAGNOSTIC AIDS
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Aids
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Aids
WARNING: This page is about a different car, the 2003 Pontiac Aztek and 2003 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.
- Operate vehicle under condition of customer's concern. Under high temperature conditions fuel vapors may increase to a point of EVAP canister vapor saturation. Fuel vapors would then be released into atmosphere. Once engine is running and EVAP purge is enabled, all fuel vapor release would be eliminated.
- Test for fuel leaking in fuel tank, rear tank if equipped, and fuel feed and return pipes.
- Test for fuel vapor leaks in fuel tank, fill limiter vent valve and rollover valves, fuel fill pipe, hose and fuel fill cap, fuel sender housing and seal, Fuel Tank Pressure (FTP) sensor seal and EVAP vapor pipe.
- Movement of EVAP pipes or fuel pipes may help find an intermittent condition.
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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.