A few years ago I had a motor start to run a little hot, but not to the extent it would boil over. I got to trouble shooting it and checked all the usual items that normally cause this problem and came up with nothing. After the problem became a little worse I decided to remove the water pump and have a look at it; I found all the impellers corroded down to little nubs that would barely move any cooant at all. I had clean coolant and no signs of corrosion in my radiator, just corrosion on the pump impellers. The pump also did not leak or have bad bearings; only way to see it was failing was to remove it.