Block doesn't drain fully. Water/coolant remains in bottom of block, especially if you only drain via the radiator and not via the block drain on exhaust side of block. Block drain is around cyl. # 3 or 4, about same height as the oil filter. Takes a 13 mm. wrench, and maybe a wire poke to clear it's passage.

I always try to add 5 qt of anti-freeze after flushing with water as I know the total will be 10 qt. That way the mix is 50-50, regardless of how much water was in there before I add the ant-freeze. This works as long as you know that whatever is left in there from the flush is water.
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Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).