How do you know when a radiator needs flushing?

Radiator fluid or coolant becomes corrosive over time and damages the cooling system. Manufacturers give recommendations for replacement, but they can be confusing. Because coolant depletion is a chemical reaction, time if far more important than mileage. With a five year or 100,000 mile recommendation, five years are the key.

Testing the pH of coolant gives a pretty good idea. New coolant has reserve alkalinity and measures 8.5 or higher. When coolant reaches 7.0 the reserve alkalinity is depleted and damage may begin. This is a good time to replace.