Consequences of coolant phase change in radiator

Normally a phase change of coolant in an engine is either the conversion of liquid coolant to gas (boiling), or the conversion of liquid coolant to solid (freezing).

Most liquid cooled internal combustion engines are water based coolant systems. And an unfortunate characteristic of water is that when it freezes (solidifies) there is an increase in volume. This increase involves potentially high forces and can crack engine blocks, split open radiators and cause coolant plumbing and cabin heaters to fail. To counteract this, ethylene or propylene glycol are commonly used. Mixtures will depress the freezing point and have the side benefit of increasing the boiling point. A common mixture is 50/50 ethylene glycol and water.

If an engine has been subjected to freezing, there may be structural damage to hard components. If an engine has been subjected to overheating and water (and other additives) have boiled off, then softer components like heater hoses, radiator hoses and even radiator and heater cores may be damaged, and need replacement.

Preventative measures include periodic cooling system inspections, and measuring the concentration of antifreeze agents with the water. This is done with hydrometers to measure the density (commonly a float in a drawing tube. It can also more accurately be measured with a portable refractometer. Both are relatively inexpensive, although the refractometer is just a little more difficult to master.