Why can't I drain my coolant?

[Squid Gloves]

07 Speed3 with 182K miles. I'm trying to flush and replace the coolant since I just replaced the stock oil cooler and I have no way of knowing what coolant the previous owner used, but I can't get the coolant to drain properly. With every car I've owned before, I've been able to drain all the coolant via the radiator, cycle a few gallons of distilled water through everything, repeat a couple times and refill the coolant, so I assumed it would be the same process on this car.

I tried draining the coolant from the radiator first with the reservoir cap off, but hardly anything actually came out. Even when I disconnected one of the oil cooler hoses, only about a gallon came out. I added water back into the reservoir and did the bleed procedure and went to drain it, and this time it trickled out a little less than a gallon after about 15 minutes. After the third time doing this, nothing came out of the radiator. So now I'm running probably 80:20 water:coolant, and I can't get it to all dump out so I can run 50:50. What am I doing wrong here?

The car is also driving fine and won't go above halfway on the temp gauge. I drove aggressively around my area for about 5 miles and I couldn't get it to overheat or behave weirdly. I'm in socal so I'm not concerned with freezing temperatures, but I'm still concerned about the fact that I can't fully drain it.

 

[Enki]

The whole thing only holds like 2 gallons, and you also need to factor in the heater core, which can hold a good portion of that. Maybe jack the back of the car up in the air (chock the fronts and put it in first though before you do this) and that might help drain the rest. Also, if you go key on (no start) and then turn the heater on, that *might* do something but I'm not sure.