2006 Ford Explorer Running hot

If there are no obvious leaks or wet spots under the engine, the most likely cause is a leaking cylinder head gasket. To verify that, you can add a small bottle of dark purple dye to the coolant, then search 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'll find the dye inside the tail pipe.

You can also have a chemical test performed at the radiator to identify a leaking head gasket. That 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 sneaking into the cooling system, the liquid will turn bright yellow.