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 283 (ENGINE CONTROLS - TROUBLESHOOTING - NO CODES)SYMPTOMSHESITATION, SAG OR STUMBLESENSOR/SYSTEM
1999 GMC Forward Control 5.7 R, Automatic
Hesitation, Sag Or Stumble: Sensor/System
1999 GMC Forward Control 5.7 R, AutomaticSECTION Sensor/System
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.
- Inspect the Heated Oxygen (HO2S) sensor for proper operation. The sensors should respond quickly to different throttle positions. If not, inspect the sensors for silicon from RTV sealant, or other contaminants from the fuel. The sensors may have a white powdery coating. Silicon contamination causes a false high signal voltage (rich exhaust indicated). The Powertrain Control Module (PCM) will reduce the amount of fuel delivered to the engine.
- Test the Manifold Absolute Pressure (MAP) sensor for proper operation. See DTC P0106: MAP SYSTEM PERFORMANCE under DIAGNOSTIC TESTS in SELF-DIAGNOSTICS - 4.6L DEVILLE & SEVILLE article.
- Test the throttle body, Throttle Position (TP) sensor, and accelerator cables for proper operation. See DTC P0121: TP SENSOR PERFORMANCE under DIAGNOSTIC TESTS in SELF-DIAGNOSTICS - 4.6L DEVILLE & SEVILLE article.
- Test the Exhaust Gas Recirculation (EGR) system for proper operation. See EXHAUST GAS RECIRCULATION under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - 4.6L DEVILLE & SEVILLE 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.