A thermostat can be tested by removing it and placing it in a pot of water. Bring the water to a boil on the stove; if the thermostat opens just before the water begins to boil, it is functioning. If the water reaches a boil and the valve has yet to open, it has failed. However, testing a thermostat is largely a waste of time. It is an inexpensive part and if you're going through the trouble of removing it, it might as well be replaced.
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With the engine completely cooled...
• Place a suitable drain pan beneath the radiator drain petcock (passenger side of radiator for 5.0L, 5.8L Windsor engines) and drain 1 to 2 gallons of engine coolant from the radiator after removing the radiator cap. Close the radiator petcock once an appropriate amount of coolant has been drained.
• The upper radiator hose connects to the inlet of the thermostat housing. Remove the upper radiator hose and any intermediate coolant lines from the thermostat housing. Do not damage the hoses or they will have to be replaced. Place a drain pan beneath the housing to catch any fluid that may spill when the hoses are removed.
• Remove the thermostat housing, which is held in place by two hex head bolts (1/2" socket required). Use a small screwdriver or pry bar to remove the thermostat housing should the gasket stick. Do not mar the mating surfaces.
• Make note of the position of the thermostat itself, as the new one must be installed in the same configuration for it to work properly.
• Remove any gasket debris that may have found its way into the coolant passage of the engine block, then stick a clean rag in the passage to prevent further contamination. With the rag in place, clean any old gasket material off of the block where the thermostat housing mounts. The surface needs to be clean and dry before the housing is reinstalled.
• Clean the thermostat housing of any gasket material or grime that may prevent the new gasket from sealing. We used a gasket scraper and some emery cloth, then cleaned it thoroughly with brake cleaner.
• With all mating surfaces clean, install the new thermostat into the thermostat housing, being cautious not to position it backwards.
• Remove the rag from the coolant passage in the engine block! Make sure all mating surfaces are dry before proceeding to the next step.
• Reinstall the thermostat housing with a new gasket and the thermostat installed. Silicon or additional "liquid gaskets" are not necessary if the mating surfaces are not damaged. However, a light coat of RTV silicon can be applied for good measure. If using silicon, let it cure per the manufacturers recommendations before refilling the radiator.
• Reinstall and tighten down the thermostat housing bolts. Snug these bolts, but do not overtighten since the threads are aluminum.
• Reinstall the upper radiator hose, any intermediate hoses that were removed, and then refill the radiator with coolant.
• Start the truck and, if there are no leaks, allow it to reach operating temperature, then shut it off and let it cool. Once the engine has cooled, check the coolant level and add as necessary.