I wonder how many folks got caught off guard by the super cold snap (especially down south) and wound up with an engine block cracked from freezing??
I've heard with "some" antifreeze in a vehicle, the coolant will just get mushy - even if the temp gets below what a tester actually shows for the coolant. Not sure if thats true of all antifreeze types or not.
I had a big stationary engine years ago that I knew didn't have enough antifreeze - so I was over there in the dark that night of the first big cold snap trying to drain it before it got too cold. Got coolant draining out of the jacket draincocks and thought I was ok - but later found it went ahead and cracked one side... I was able to stitch it up with some nickel rod, but always have wondered how it was able to drain coolant and then still freeze tight enough to crack...