Overheating

When there is no visible coolant leak, a leaking cylinder head gasket is the most common suspect, but that was not common on your engine. There is two ways to test for that. One is to add a small bottle of dark purple dye to the coolant, then check a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If a head gasket is leaking, you will find the dye inside the tail pipe. Auto parts stores will have the dye for the fluid you are checking, and those that rent or borrow tools should have a black light.

The second 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. The liquid will turn bright yellow if combustion gases are present. You will need a mechanic to do this test. You can borrow the tool from some auto parts stores, but they will make you buy your own bottle of fluid. That is because it is rendered ineffective if it gets contaminated with antifreeze. It would not work properly for the next person. This test only takes a minute or two to perform.