A: Some coolants are sensitive to hard water, which can cause some corrosion inhibitors to form insoluble salts in the water. The result is premature coolant failure. Hard water salts will deposit on hot areas within the coolant system creating insulating films, which contribute to overheating problems in the coolant system.
Deionized water is highly recommended for flushing your cooling system and diluting engine antifreeze/coolant because it is purified water; it doesn’t contain any mineral ions such as sodium, iron, copper and chloride that can cause corrosion. Deionized water reduces corrosion in the cooling system and helps to decrease maintenance requirements of engine radiators. Using this purified water in the vehicle’s system provides protection against the damaging effects of scale build-up and corrosion caused by the use of regular tap water.