Toyota 90916-03140 Thermostat

Disconnect the cable from the negative battery terminal. Raise the front of the vehicle and support it securely on jackstands, then remove the lower engine covers and the lower inner fender corner splash shield(s). Drain the engine coolant and disconnect the radiator hose from the thermostat housing or water inlet pipe. For four-cylinder engines, on 2010 and later models, remove the alternator. Disconnect the radiator hose from the thermostat housing, then unbolt the thermostat housing from the engine. Remove the thermostat, noting the direction in which it was installed in the housing, and thoroughly clean the sealing surfaces. Install a new gasket to the thermostat, ensuring it is evenly fitted all the way around. Install the thermostat into the engine with the jiggle valve positioned at the highest point, then install the thermostat housing. For V6 engines, remove the front engine mount bracket and control rod assembly, the drivebelt, and the idler pulley assembly. Disconnect the radiator hose from the thermostat housing, then remove the thermostat housing mounting fasteners and housing from the coolant inlet housing. Remove the thermostat, noting the direction in which it was installed in the housing, and thoroughly clean the sealing surfaces. Install a new gasket to the thermostat and place it into the inlet housing with the jiggle valve positioned at the highest point, then install the thermostat housing. Tighten the housing fasteners to the specified torque and reinstall the remaining components in the reverse order of removal. Refill the cooling system, run the engine, and check for leaks and proper operation.

Disconnect the bypass hose and radiator hose from the thermostat housing, then remove the thermostat housing mounting bolts.

Before assuming the thermostat is the issue, you should check the coolant level, drive belt tension, and temperature gauge (or light) operation.