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.
1999 GMC Forward Control 5.7 R, AutomaticFACTORY OEM SERVICE PROCEDURE
Function Check
- With ignition on, engine not running and at room temperature, glow plugs are on for 4-6 seconds, and then off for about 4.5 seconds. Following this on-and-off cycle, glow plugs are on for about one second and off for about 3 seconds. The glow plugs continue to cycle on and off in this time sequence for about 16 seconds.
- If engine is cranked during or after the above sequence, glow plugs will cycle on and off for a total duration of 25 seconds after ignition switch is returned from crank position, whether engine starts or not. Engine does not have to be running to terminate glow plug cycling.
Factory Service Tools
Use the Manual With the Right Hardware
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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.
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