Why is there dust at the bottom of the overflow tank?

Answer 1

If the bottom of the overflow tank has dust as in "dry as dust," this suggests there isn't enough coolant in the system. Most cars are designed to always have coolant in the tank, with "hot" and "cold" fluid levels marked on the side.

If coolant is added to the tank and disappears over a few weeks (or days!) of driving, that indicates a leak somewhere in the cooling system -- either the tank, the connecting tube or somewhere else.

If the dust is more like dirt, rust particles or miscellaneous junk that has settled to the bottom of the liquid in the tank, that's not unusual. It's a good idea to clean out the tank every once in a while to make sure the tube to the radiator doesn't get clogged.

If there's a LOT of debris in the overflow tank, it's likely the rest of the cooling system is also dirty and needs to be flushed. This can be done by a service station or using a garden hose and a radiator flush kit available at auto parts stores. The do-it-yourself version is usually not quite as effective, but much cheaper.