Position a suitable drain pan under the radiator drain cock and drain the cooling system.
On the 1MZ-FE engine, remove the air cleaner cap assembly. Be sure to label all wiring disconnected.
Disconnect the water temperature switch harness from the water inlet housing (except 5S-FE engines).
Remove the oil filter on 5S-FE engines. On the 1MZ-FE engines, remove the heater hose covering the housing.
Loosen the hose clamp and disconnect the lower radiator hose from the water inlet housing.
Remove the two nuts from the water inlet housing and remove the housing from the water pump studs.
Remove the thermostat and rubber O-ring gasket from the water inlet housing.
To install: Make sure all the gasket surfaces are clean. Clean the inside of the inlet housing and the radiator hose connection with a rag.
Install the new rubber O-ring gasket onto the thermostat. On 2SE and 3S-FE engines, align the jiggle valve of the thermostat with the protrusion on the water inlet housing. On 5S-FE engines, align the jiggle valve with the upper side of the stud bolt. On 2VZ-FE and 3VZ-FE engines, align the jiggle valve with the stud bolt. Insert the thermostat into the housing.
Position the water inlet housing with the thermostat over the studs on the water pump and install the two nuts. Tighten the two nuts to 78 inch lbs. (9 Nm) on 4 cyl. Engines, 2VZ-FE and 3VZ-FE; 14 ft. Lbs. (20 Nm) and 69 inch lbs. (8 Nm) on 1MZ-FE engines.
Don't forget to install a new O-ring onto the water inlet pipe. Apply soapy water to the O-ring prior to installation.
Connect the lower radiator hose to the inlet housing and install the hose clamp. On the 3VZ-FE, tighten the bolt holding the water inlet hose to the alternator belt adjusting bar to 14 ft. Lbs. (20 Nm). On the 1MZ-FE engine, tighten the bolt retaining the water inlet pipe to the cylinder head to 14 ft. Lbs. (20 Nm).
Attach the water temperature switch connector.
On the 1MZ-FE engines, install the heater hose removed.
Install the oil filter and check the oil level if removed. Install the air cleaner cap assembly on the 1MZ-FE engines.
Fill the cooling system with a good brand of ethylene glycol based coolant.
Start the engine and inspect for leaks.