In the old days, cooling systems typically ran water in the summer and a mixture of methanol and water in the winter. Methanol (methyl alcohol) was a problem with the early vented radiator caps because it would boil away with use and would have to be constantly replenished to ensure that it had a low freezing point. While a 35% (vol) concentration of methanol has a freeze point of -15°F/-26°C, it also gives off toxic fumes and has a flash point of 95°F/35°C. The flash point is the temperature at which it will generate a combustible fuel mixture.

Ethylene glycol is added to water to both raise engine coolant's boiling point and lower its freezing point. Ethylene glycol and water mixtures (aqueous ethylene glycol) has a lower freezing point than either pure water or pure ethylene glycol and it has as eutectic temperature of about -69°C/-92°F at about 68% ethylene glycol & 32% water by volume. A common concentration of automotive coolant is 50% ethylene glycol by volume, which provides a freeze point of -34.2°F/-36.8°C. Below this temperature, the ethylene glycol solution with be a slush rather than a solid.

Water is an excellent heat transfer fluid as it has a specific heat capacity about twice that of ethylene glycol so that pure liquid water is twice the ability of pure liquid ethylene glycol to transfer heat. Using 50% ethylene glycol coolant requires about 25% more flow to carry away the same amount of heat as pure water.

Ethylene glycol generally does not degrade with use. The stabilizers and corrosion inhibitors are gradually consumed over time which is why the coolant must be periodically replaced. The old "green" coolants typically have a lifespan of 2-3 years while newer long-life coolants (OAT -Organic Acid Technology, HOAT - Hybrid Organic Acid Technology) typically last 5 years. Do not mix coolant types unless the product you are adding specially states that it is compatible with the coolant in your system.

It is better to use distilled water in the cooling system rather than tap water because tap water contains impurities such as chlorine and minerals.  Chlorine is used for disinfection but can contribute to corrosion.  Naturally occurring minerals in tap water can become scale in the cooling system.

ETHYLENE GLYCOL POISONING.Ethylene glycol tastes sweet but is very toxic to humans and animals. Clean up all spills to prevent ingestion!