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 219 (AUTOMATIC TRANSMISSION SERVICING)LUBRICATIONOIL COOLER FLUSHING & FLOW CHECKBACK FLUSH
1999 GMC Forward Control 5.7 R, Automatic
Back Flush
1999 GMC Forward Control 5.7 R, AutomaticSECTION Back Flush
WARNING: This page is about a different car, the 2003 GMC Sonoma, 2003 GMC Jimmy, 2003 Chevrolet S10 Pickup, and 2003 Chevrolet Blazer. However, it is still accessible from the selected car via links, so may be relevant.
- Inspect the transmission oil cooler pipes for kinks or damage. Repair as necessary.
- Connect the transmission oil cooler and line flusher to the oil cooler feed bottom connector. Use the Cooler Flushing Adapter (J 35944-200), if required.
- Clip the discharge hose (5) onto the oil drain container. See Fig 1.
- Attach the transmission oil cooler and line flusher to the undercarriage of the vehicle with the hook provided and connect the flushing system feed supply hose (3) from the transmission oil cooler and line flusher to the top connector oil cooler return pipe. Use the cooler flushing adapter, if required.
- 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. 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 onto a 5 gallon pail with a lid, to avoid splashback.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 bucket or similar container.
- 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. Use the clip provided on the handle to hold the trigger (8) down. See Fig 2. 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 120 psi (825 kPa) 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.
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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.