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FIRST have enough replacement coolant on hand by computing the capacity of the system times one-half (at fifty-fifty). The heater core will store some coolant or water, so figure that in. If you flush it with water, then you have to add fifty percent of the capacity in 100% coolant to reach a 50-50 mix. If you merely drain it, the heater will hold the 50-50 mix already in it, so you will need less new coolant.
Second, isn't now a good time to replace the hoses and the heater water valve? They are twelve years old, they don't last forever, and by replacing them you gain peace of mind that the engine will not toast itself. "Hoses" mean radiator, heater, and reservoir hoses.
Third, if you want to flush, then remove the thermostat and replace the housing, open the heater temp valve to "hot", and drain thoroughly through the bottom radiator hose and both front and rear block drain plugs. Refill with tap water. Run the engine for a minute or so. Drain and refill, doing this until it runs clear. For your last refill flush, use distilled water. Then drain one last time. Fill with pure coolant until you have enough in the system to equal 50-50 mix. Begin topping off with distilled water until it bubbles out of the thermostat housing. Replace the thermostat, with the air bleed hole up, button it up, and continue topping off. Run the engine until warm and let it cool down. Top off the remaining amount, again with distilled water. Check the reservoir a day later and top off again.
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