12 midnight, 85 degrees. Driving home, the temp gauge jumps to hot (red zone). Notice some steam in the engine compartment. Stop and shut off the engine. This or similar has not happened before with this car.
Overflow tank (not pressurized) had reasonable amount of coolant, not high not low. No coolant on ground under car, no drips. No visible cooant in engine, no coolant odor. Radiator had no visible coolant but only took maybe 2 cups. Radiator fan works as required. Engine runs and sounds good.
I think of a defective thermostat as a possible cause. Cooling systems are not particularly complex - what else to consider?