Originally Posted by Externet

Hi.
A 180F thermostat changes coolant flow path from recirculating on the bypass hose to circulate towards the radiator a couple of minutes sooner than a 190F thermostat.
A 180F thermostat is to not allow the engine to run colder than 180F.
A 180F thermostat does not make the engine to run cooler than a 190F thermostat.
A 180F thermostat does not mean the coolant will be under nor at 180F.

After the thermostat has changed the coolant flow path from bypass to radiator at its rated temperature, the thermostat does nothing. Absolutely nothing, regulates nothing, does not move a bit. Nothing, until hours after the engine is stopped when the coolant temperature returns to below its rated temperature. Very good when you drive in Alaska and nearby.

An any temperature in-line thermostat works without the bypass path; just keeps closed the coolant flow from engine towards radiator until coolant reaches its rated temperature and opens flow. Then it does nothing, absolutely nothing, is open to flow and cannot open more.