You have to let it drain completely. If your going to do a full cleaning, they sell some detergent stuff at the auto store. You throw that in and fill it w / water and let it drain again. Make sure you get all the stuff out. Then put the drain plug back in and add 50% coolant/50% water. I would also recommend Water Wetter. You can get it from your auto parts store too. Keeps thing a little cooler.
If you only drain the radiator and refill it, you're leaving almost half the coolant in the car though. Shouldn't you buy one of those adapters that you put inline with one of the radiator hoses so you can flush the coolant in the engine out with a lawn hose hookup and water? I would think that only changing half the fluid would not be that effective and you would also not be flushing the engine coolant passages out. Mine is about due, so which method do most of you guys use?
The adapter to flush is just a T connector that lets you hook a hose up and use water to flush 100% of the coolant out. It's like 5 bucks at K-Mart. I just want to be sure that it will work with a modular motor like it did with my 350.
I have yet to do it on my mustang, but what I always do on other cars is drain the rad. Disconnect upper and lower rad hoses from rad, leave the lower one hooked up to the block but disconnected from rad. Pull thermostat housing and thermostat itself. Start the car, have someone keep an eye on your temp guage, take the garden hose and spray water in the lower rad hose. This pushes all of the old coolant through the block, with the help of the water pump, and it all comes out the T-stat area. I just keep doing it until it runs clear, normally about a minute. Then just replace hoses and T stat and refill rad with 50/50 mix...do not replace rad cap yet. Start motor again and fill rad to the top as necessary. Hose any coolant off of the motor and you're done. I've never had any problems with this method and it's better than hacking up your heater hoses to put in a flushing tee (adapter for garden hose).
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