Overheating after Thermostat Change out

Could it be your old thermostat was stuck open a bit, never letting engine warm up fully? Now with new unit installed, opening and closing as is should, has unmasked a partially plug radiator or other cooling system deficiencies. If you suspect a lower coolant hose collapse impeding flow, that is a symptom of a plugged radiator restricting coolant flow.

I can tell you from experience that when thermostat is installed backwards, the engine heats up real fast to the point of overheating because the spring that senses hot water is parked on the cool side of the system, and can’t react to engine heat.