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 MANUALSGMC1991FORWARD CONTROL 5.7 K, AUTOMATICREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - SYSTEM/COMPONENT TESTS
1991 GMC Forward Control 5.7 K, Automatic
Engine Controls - System/Component Tests
1991 GMC Forward Control 5.7 K, AutomaticSECTION Engine Controls - System/Component Tests
- Introduction
- Air Induction Systems
- Computerized Engine Controls
- Control Unit
- Engine Sensors & Switches
- A/C On Switch System Test
- Brake Switch
- Coolant Temperature Sensor (Cts)
- Knock Sensor
- Manifold Absolute Pressure (Map) Sensor
- Manifold Air Temperature (Mat) Sensor
- Oxygen (O2) Sensor
- Oxygen Sensor Heating Element (4.3L Turbo PFI)
- Park/Neutral (P/N) Switch
- Power Steering Pressure Switch (2.5L)
- Throttle Position Sensor (TPS)
- Vehicle Speed Sensor (Pm Generator Type)
- Relays, Solenoids, Motors & Modules
- Fuel System (Gasoline)
- Fuel System (Diesel Glow Plug System Diagnosis)
- Ignition System
- Emission Systems & Sub-Systems (Gasoline)
- Emission Systems & Sub-Systems (Diesel)
- Miscellaneous ECM Controls
- Idle Air Control (IAC)
- Component Locations
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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.