Where is the thermostat on a 1991 Ford F-250 diesel?
The 1991 Ford F-250 diesel doesn't have a thermostat in the same way a modern car with climate control does. Instead, it refers to the engine coolant thermostat. This is located in the engine's water pump housing. You'll need to consult a repair manual specific to your 1991 F-250 diesel engine (either the 7.3L IDI or 6.9L IDI, depending on your truck) to locate it precisely as the access varies slightly depending on the engine. Be prepared to get your hands dirty; it's not typically easy to access.