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 69 (AUTOMATIC TRANSMISSION SERVICING)LUBRICATIONOIL COOLER FLUSHINGINITIAL FLUSH
1999 GMC Forward Control 5.7 R, Automatic
Initial Flush
1999 GMC Forward Control 5.7 R, AutomaticSECTION Initial Flush
WARNING: This page is about a different car, the 2002 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
- Turn the transmission oil cooler and line flusher water supply valve (1) to the ON position and allow water to flow through the oil cooler and pipes for 10 seconds to remove any remaining transmission fluid. See Fig 1 . If water does not flow through the oil cooler and pipes, the cause of the blockage must be diagnosed and the plugged component must be repaired or replaced. Continue with the cooler flushing and flow check procedure once the blockage is corrected.
- Turn the transmission oil cooler and line flusher water supply valve (1) to the OFF position and clip the discharge hose (5) onto a 5 gallon (19L) pail with a lid, to avoid splashback. See Fig 2 . Turn the transmission oil cooler and line flusher water supply valve (1) to the ON position and depress the trigger (8) to mix cooler flushing solution into the water flow. See Figure . Use the clip provided on the handle to hold the trigger (8) down. The discharge will foam vigorously when the solution is introduced into the water stream.
- Flush the oil cooler and pipes with water and solution for 2 minutes. During this flush, attach the shop air supply of 120 psi to the flushing system feed air valve (2) located on the transmission oil cooler and line flusher for 3-5 seconds at the end of every 15-20 second interval to create a surging action.
- Release the trigger (8) and turn the transmission oil cooler and line flusher water supply valve (1) to the OFF position.
NOTE:
Flushing for approximately 2 minutes in each cooler line direction will result in a total of about 8-10 gallons of waste fluid. This mixture of water and flushing fluid is to be captured in a 5 gallon bucket or similar container.
RENDER: 1.0x
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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.