The thermostat controls the flow of coolant to the engine using its heat-sensitive valve. When your Nissan's engine reaches a certain degree, the thermostat opens to allow cold coolant from the radiator into the engine. The super-heated coolant already within the engine automatically returns to the radiator for cooling. During normal operation, this process repeats until you turn the engine off in your Quest. A jammed or defective thermostat will cause your Quest's engine to overheat and it may suffer mechanical failure as a result. The thermostat in the Nissan Quest mounts within the engine compartment. You can replace it at home with a few tools if you have determined that it's defective.
Pull your Nissan Quest's hood release lever and raise the hood. Locate the radiator's drain cock on the inner, lower corner of the radiator. Position a coolant drain pan under the radiator, directly beneath the drain cock.
Open the drain cock slowly by turning it to the right until you see that your Quest's coolant is starting to flow out of the radiator. With the drain cock open, count to 45 and then close the drain cock once more. You only need to drain 25 percent of the coolant.
Locate the top radiator hose connected to the upper, passenger side of your Quest's radiator. The hose looks just like the lower radiator hose that sits next to the drain cock. The top radiator hose connects to the radiator and extends another 2 feet to where it connects to the side of the engine. At this point you will see an additional hose that curves away sharply and connects to a round, metal housing. This is the thermostat housing.
Loosen and remove the four bolts securing your Quest's thermostat housing to the engine with a socket wrench. Hold one hand beneath the housing while you pull it away from the engine. The thermostat should drop right into your hand.
Scrape any remaining RTV sealant from both flat, connecting points with a plastic putty knife. Once cleaned, work quickly to reapply a generous amount of RTV sealant around the entire flat, connecting surface of the thermostat housing, only.
Insert your Quest's new thermostat into mounting position within the engine. Place the thermostat housing back over the thermostat and begin threading all four of its securing bolts by hand. Tighten all four of the housings bolts with your socket wrench until snug. Once all four of the bolts are snug, tighten them all in a star pattern and then with an additional quarter-turn with your socket wrench.
Remove your Quest's radiator cap and refill the radiator with fresh coolant or with the coolant from your drain pan (providing it was clean initially). Shut the hood and allow the RTV sealant to set-up for two full hours before you attempt to start your Nissan Quest's engine.
