first start break in

Here's a quote from a scientific website about water vs. coolant:
Anti-freeze (ethylene glycol) has several functions as a heat exchange medium in an automobile engine. One is, as you point out, to reduce the freezing point of the heat exchange fluid. But the freezing point of pure ethylene glycol is -13 C. (which is only 8.6 F.) so at first this does not seem like a very likely candidate for an "anti-freeze". But the process is more subtle. Mixtures of ethylene glycol and water freeze at temperatures less than either component. In addition, both ethylene glycol and ethylene glycol / water mixtures tend not to freeze but rather form a viscous glass instead. As an aside, glycerol has a melting point of 17.8 C., but few people have ever seen crystals of glycerol, because it too tends to form a glass rather than crystallizing as the temperature is lowered.

The boiling point of ethylene glycol is about 198 C. -- much higher than the boiling point of water -- so mixtures of the two can be used as a heat exchange fluid at temperatures significantly greater than 100 C. at significantly lower pressures than water alone (in a closed system). In principle, water alone could be used as a heat exchange fluid, but its vapor pressure doubles from 1 to 2 atmospheres at a temperature just over 120 C. So the problem becomes one of engineering hoses, fittings, etc. that could withstand high pressures if water alone were used as a heat exchange fluid.

When the term "boiling over" is used in the context of a heat exchange medium what is meant, as I am sure you know, is that the pressure of the fluid is greater than atmospheric pressure and the fluid evaporates until the pressures are equal.

In the "real world" there are a number of other factors that need to be taken into account, such as corrosion, and viscosity.

Vince Calder http://www.newton.dep.anl.gov/askasci/chem03/chem03987.htm