Why is my engine overheating?

I can't see the leak so you'll need a live person to search for that. A pressure test might be called for or I can describe how to use a small bottle of dye. The dye works best with slow, hard-to-find leaks.

For overheating, start by having your mechanic perform a chemical test at the radiator. That involves drawing air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If that liquid turns bright yellow, the cylinder head gasket is leaking. Combustion gases will seep into the cooling system and pool under the thermostat. Thermostats don't open in response to hot air. They must be hit with hot liquid.