Changing a thermostat on a 1991 Toyota Previa is a job most people can perform. The thermostat controls the flow of coolant from the radiator to the engine. When the coolant reaches a certain temperature, the thermostat opens and allows the coolant to flow from the engine to the radiator, where it is cooled. As the thermostat ages, it may stick and eventually may stay closed. At that time, the thermostat must be replaced. You may want to replace the thermostat sooner to avoid having problems while you're driving.
Follow the large hose at the top of the radiator, on the driver's side, down to the thermostat housing. The housing is located on this hose, just before it goes into the engine. Place the bucket on the ground under the thermostat housing.
Remove the two bolts that hold the housing together with the socket set. Coolant may leak out as you loosen the bolts so adjust the placement of the bucket to catch the coolant as it drains. Remove the thermostat, taking note of how it was positioned in the housing.
Remove the gasket from the housing. Use a flat head screwdriver to scrape the gasket off the housing if it is stuck. Remove all remnants of the gasket to prevent leaking.
Place the new thermostat into the housing so it points in the same direction the used thermostat was pointed. Spread the lubricant on the new gasket, and place it back in the housing.
Put the housing back together and tighten the two bolts. Start the car. Make sure no coolant is leaking from the housing. Fill the coolant reservoir up to the "full" line with a mixture that's half water and half coolant.
Discard any used coolant in an environmentally friendly way. Many auto repair shops have places to dump used coolant.
Tighten the bolts so the housing does not leak, but not so tight that the housing breaks. Never open the radiator cap or remove any part of the cooling system until the engine is at a safe temperature.