My daughter's 98 Mazda Protege uses up water. We filled the radiator, and the oveflow tank to the max fill line with coolant, let it sit a couple days. No leaks.

Yesterday we drove it about 20 miles, street and freeway. Afterwards, the radiator needed a couple cups of water, and the coolant was down about an inch. The car runs fine.

My daughter had not been checking it often, thought it "just overheats". But it uses water. When I filled it, the resevoir was empty, and the radiator took almost two quarts of water. So it likely runs very low in the course of a single tank of gas.

They already replaced the engine 4 months ago.

Slight hose leak ? Or what's a better idea