Overheating

Since this happened at highway speed, we can rule out a bad radiator fan. The next suspect is a leaking cylinder head gasket. Your mechanic can perform a chemical check at the radiator for that. The test involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

You can borrow this tool from an auto parts store that rents or borrows tools, but they will make you buy your own bottle of fluid. That is because it will be rendered ineffective if it freezes, or if it becomes contaminated with antifreeze. They do not want to risk borrowing it to the next person with contaminated fluid. It is often less expensive to pay a mechanic to do the test than it costs for a bottle of fluid. The test just takes a few minutes.