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 KNOCKADDITIONAL CHECKS
1999 GMC Forward Control 5.7 R, Automatic
Additional Checks
1999 GMC Forward Control 5.7 R, AutomaticSECTION Additional Checks
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 Crankshaft Position (CKP) sensor "B" for an internal short.
- Check for proper Exhaust Gas Recirculation (EGR) system operation. See DTC P0401: EGR FLOW INSUFFICIENT under DIAGNOSTIC TESTS in SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article.
- Check the transaxle range switch input with the vehicle in Drive and the gear selector in Drive or Overdrive.
- Check the Torque Converter Clutch (TCC) operation. TCC applying too soon can cause the engine to spark knock.
- If the scan tool readings are normal and there are no engine mechanical faults, fill the fuel tank with a known quality gasoline that has a minimum octane reading of 87 and re-evaluate vehicle performance.
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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.