Cooling system

If you start the engine cold and it's overheating within one mile, it can't get that hot that fast. What are the symptoms? If you're seeing boiling in the coolant reservoir, you'll see there's no steam associated with it, so you're seeing air bubbles, not boiling. That's a sign of a leaking head gasket. Your mechanic can perform a combustion gas test at the radiator to verify a head gasket problem.

If you're just watching the temperature gauge on the dash, feel the upper radiator hose as soon as the gauge goes to "hot". It will normally be too hot to hold onto for very long. If it's still cool, the thermostat hasn't opened up yet or the engine isn't really that hot. Be aware too that a leaking head gasket can cause an air pocket to form under the thermostat. Thermostats will not open in response to hot air. They open in response to hot liquid, so a thermostat that doesn't open can cause overheating, but it's not necessarily the thermostat that is defective.