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 261 (ENGINE CONTROLS TROUBLESHOOTING - NO CODES - 3.1L)SYMPTOMSDETONATION/SPARK KNOCKFUEL SYSTEM
1999 GMC Forward Control 5.7 R, Automatic
Fuel System
1999 GMC Forward Control 5.7 R, AutomaticSECTION Fuel System
WARNING: This page is about a different car, the 2003 Pontiac Grand Prix, 2003 Chevrolet Malibu, and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
- Check the fuel system circuits for proper operation. See FUEL PUMP ELECTRICAL CIRCUIT DIAGNOSIS under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 3.1L CENTURY, GRAND PRIX & MALIBU article.
- Check for low fuel pressure. See FUEL SYSTEM DIAGNOSIS under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 3.1L CENTURY, GRAND PRIX & MALIBU article.
- Check for faulty fuel injectors. See FUEL INJECTOR BALANCE TEST WITH SPECIAL TOOL , FUEL INJECTOR BALANCE TEST WITH TECH 2 and FUEL INJECTOR COIL TEST under FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 3.1L CENTURY, GRAND PRIX & MALIBU article.
- Check for fuel contamination. See ALCOHOL/CONTAMINANTS-IN-FUEL DIAGNOSIS W/O SPECIAL TOOL or ALCOHOL/CONTAMINANTS-IN-FUEL DIAGNOSIS W/ SPECIAL TOOL under FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 3.1L CENTURY, GRAND PRIX & MALIBU article.
- Check any items which may cause an engine to run rich, long term fuel trim is significantly in the negative range. See DIAGNOSTIC AIDS for DTC P0172: FUEL TRIM SYSTEM RICH under DIAGNOSTIC TESTS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article.
- Check any items which may cause an engine to run lean, long term fuel trim is significantly in the positive range. See DIAGNOSTIC AIDS for DTC P0171: FUEL TRIM SYSTEM LEAN under DIAGNOSTIC TESTS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article.
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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.