My radiator has been overheating rather frequently; but it only seems to happen on extended trips. The car will be running at normal operating temperature for 40 min. , and then in the space of five minutes, will suddenly overheat. When running normally, barely any fluid is lost from the radiator; but when it overheats the radiator/overflow tank loses quarts of fluid. I have had both hoses replaced, and it has been checked out several times, and no leak can be found. Also, the fans don't seem to kick on when the car overheats, but according to the auto repair shop, they do work. What could be causing the car to overheat, and why is it losing fluid?

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1991 Acura Integra honda