Coolant does more than keep your engine from overheating; it also stops it from freezing, lubricates the water pump and fights corrosion inside the cooling passages. Over time it wears out, and ignoring it leads to some of the most expensive failures a car can suffer.
Why Coolant Goes Bad
Fresh coolant contains additives that prevent rust and scale. Those additives deplete with heat and miles, and the fluid can turn acidic, eating at the radiator, water pump and gaskets from the inside. Old coolant also loses its ability to transfer heat well, so the engine runs hotter than it should even when nothing looks wrong.
Signs It's Due
- Color: bright green, orange or pink fading to a rusty brown means it's degraded.
- Debris: flecks or sludge in the reservoir signal internal corrosion.
- Temperature: a gauge creeping higher than normal in traffic.
Flush vs Top-Off
Topping off a low reservoir is fine in a pinch, but it doesn't remove old, contaminated fluid or the corrosion it carries. A proper flush drains the system, often circulates a cleaner, and refills with the correct coolant mixed to the right ratio, usually about half coolant and half distilled water. Mixing incompatible coolant types can form a gel that clogs the system, so always use the type the manufacturer specifies.
How Often
Intervals vary widely with the coolant chemistry, from around 30,000 miles for older green formulas to 100,000 miles or more for modern long-life types. Check the manual rather than guessing. A coolant flush is inexpensive next to a new radiator, water pump or, worst of all, a head gasket. Keeping the cooling system fresh is quiet, cheap insurance against an overheating engine on a hot day in traffic.