I had a stuck open thermostat on our 850 once which I didn't notice until one day I came home down a local mountain road where you spend about 10 miles descending. By the bottom of the mountain the temperature gauge was almost sitting at the bottom of it's range. Thus I deduced that the thermostat had failed open. A new one returned everything to normal.

People are very confused about thermostats in automotive cooling systems. When working correctly the thermostat will adjust itself to try and keep the engine coolant at the rated temperature of the thermostat. If the cooling system isn't able to dissipate all of the excess heat then the thermostat will remain full open and the engine temperature will rise. Under most circumstances the cooling system has excess dissipative capacity and is essentially throttled by the thermostat.

Removing the thermostat is a bad idea, yet some people still do so think that they will improve the engine's cooling capacity.

John