I've done several coolant flushes on previous cars. I tried to do it on my 2003 Mazda 6s but it obviously was unsuccessful.

Here's what happened:Only 1/4 of the expectedcoolant came out in the 1st place. I fed a garden hose into the coolant reservoir, turned it on and let the engine warm up in hopes of flushing all the old coolant out. I also turned the heater on full blast to try and circulate the water through the system. Clear liquid came out the radiator drain hole. Then when I went to fill it up, it again only took about 1/4 of the coolant that I expected to put in.
I know that it must have been hiding a bunch of the coolant from me because after I drove it, the reservoir was filled with the original colored coolant.

What's special about the Mazda 6's cooling system and how can I do acomplete flush?

 

Sounds strange. With the engine stopped, did you also open the radiator cap whenthe drain plug is opened?

 

Yes. I also tried to let any extra air out, but I still don't know why there was approx. 3/4 of the liquid that didn't fall out...?

 

Hmm, may be you also have to take out the thermostat to fully drain the coolant inside the engine.