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5.8L F-150 Thermostat Replacement

How to Replace the Thermostate on a 5.0L/5.8L V-8





The thermostat is a mechanical valve that controls the flow of coolant through the engine block. When engine coolant reaches a predetermined temperature, the thermostat opens, increasing coolant flow through passages in the engine and therefore keeping engine temperatures within the recommended range. A mechanical thermostat requires no electrical input to open and close. Over time, thermostats corrode and begin to behave erratically or fail completely, and require replacement. If your engine is running hotter than normal, the thermostat is the likely culprit. Thermostats can be tested by removing them and placing them in a pot of water. Bring the water to a boil on the stove top. If the valve opens before the water begins to boil, it is functioning properly. If the water reaches a boil and the valve does not open, its bad.


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• Begin by draining some coolant from the radiator to reduce coolant spillage when the upper radiator hose is removed. Attach a small section of fuel line or vacumm hose to the radiator petcock. The petcock is located on the passenger side of the radiator, near the bottom.
• Remove the radiator cap, and then turn the petcock to begin draining the radiator. We removed about 1 gallon of coolant just to be on the safe side. Be sure you are capturing the coolant in a container so that it can be disposed of (or reused if the container is clean).
• Once you think you have drained an appropriate amount of coolant, close the petcock.

• The thermostat housing is connected to the end of the upper radiator hose. Follow the upper hose from the radiator, to the engine and you've found your thermostat housing.
• Remove the upper radiator hose and any intermediate coolant hoses (there should also be a hose that connects the water pump to the thermostat housing) by loosening the clamps and slipping them off. Try not to damage the hoses; if they are proving difficult to remove rotate them slightly side to side as you pull.

• With the coolant hoses removed, remove the thermostat housing. It is held in place by 2 bolts that require a 1/2" socket. A small screwdriver or pry bar may be used if the housing sticks to the engine block, but do not mar the mating surfaces or it may leak when reinstalled.

• Make note of the position of the thermostat itself, as the new one must be installed in the same configuration in order to work properly (see photo if in question).

• Remove any gasket debri 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 new items are installed.

• 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 new housing gasket and new thermostat installed. Silicon or additional "liquid gaskets" are not necessary.
• Tighten down the thermostat housing bolts until they are tight. Snug these bolts, but do not overtighten since the threads are aluminum.
• Reinstall upper radiator hose, any intermediate hoses that were removed, and then refill the radiator with coolant.

• Start the truck and allow it to reach maximum operating temperature, then shut it off and let it cool. Once the engine has cooled, check the coolant level and add as necessary.