How to Replace the Thermostat on a 91 Nissan Pickupby Kevin Mclain
The cooling system on the 1991 Nissan pickups come equipped with a thermostat that regulates the engine's temperature. The thermostat is made up of a temperature-sensitive spring that moves the thermostat up and down in reaction to the temperature of the coolant. When the engine is running, the thermostat regulates the coolant flow from the radiator to the engine, allowing only the proper amount of coolant throughout the engine block. If the thermostat sticks, replace it as soon as possible.
Park the 1991 Nissan pickup on a level surface. Set the parking brake and open the hood.
Locate the top radiator hose that runs from the top of the radiator to the engine block. Follow the top radiator hose from the radiator to the thermostat housing. The hose is attached to the thermostat housing tube. Slide the fluid catch pan underneath the engine in the general area of the thermostat housing.
Loosen the hose clamp by turning the adjustable screw head counterclockwise with a flat-head screwdriver. Pull the clamp about 6 inches up onto the radiator hose. Twist the hose back and forth with your hand until it loosens from the thermostat housing tube. Pull the hose straight off the tube.
Loosen and remove the two mounting bolts from the thermostat housing with a ratchet and a socket. Pull the thermostat housing off the engine block and place it on a clean rag.
Pull the thermostat straight up out of the engine block. Scrape any of the old gasket off the base of the thermostat housing and the engine block with a flat scraper. Wipe the thermostat housing base and the engine block clean with a clean rag.
Place the thermostat gasket over the bottom of the thermostat housing base and push the two housing bolts through the holes to hold the gasket in place. Insert the new thermostat into the top of the engine block with the spring facing downward. Lower the housing back over the engine block. Screw the two mounting bolts into the engine block until tight. Finish tightening the bolts with the ratchet and socket.
Push the top radiator hose back over the thermostat housing tube. Position the hose clamp over the hose and over the thermostat housing tube. Tighten the clamp with the screwdriver until the hose is tightly secured to the thermostat housing tube.
Remove the radiator cap and add coolant as needed until the radiator is full. Crank the engine and let it run until the engine is at operating temperature or until the coolant level drops inside of the radiator. This will show that the thermostat is working properly. Inspect the thermostat housing and the top radiator hose for any leaks while the engine is running. Add more coolant if needed to fill the radiator up with the engine running.
Turn the engine off and pull the fluid catch pan out from under the truck.
- "Nissan/Datsun Pick-up & Pathfinder Haynes Repair Manual for 1980 thru 1997"; John Haynes; 1998
- Be sure to tighten both of the thermostat housing bolts evenly to properly seat the thermostat gasket to the engine block.
- Use red high temperature silicone if needed to stop any leaks around the thermostat housing gasket.
Things You'll Need
- Fluid catch pan
- Flat-head screwdriver
- 3/8-inch drive ratchet
- 3/8-inch drive socket set (metric)
- Flat metal scraper
- Clean rags
- New thermostat with gasket
- Never attempt to remove the radiator cap from a hot radiator. This can result in serious burns to the hands, arms or face. Always wait for the engine to completely cool down before removing the radiator cap.
Kevin Mclain has more than 20 years of automotive, home improvement and landscaping experience. He has been writing for various online publications since 2002. Mclain has U.S. Army certification in automotive maintenance and repair, among more than 15 additional certifications related to the automotive field.