Where is the thermostat on a 1991 Ford F-250 diesel?

On a 1991 Ford F-250 diesel, the thermostat is located in the engine's coolant outlet housing, which is typically found on the front of the engine, near the top. To access it, you'll need to remove the upper radiator hose connected to the housing. Once the hose is detached, you can unbolt the housing to reveal the thermostat inside.