Position a suitable drain pan under the radiator drain cock and drain the cooling system.
On the 1MZ-FE engine, you may have to 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 that are equipped with an oil cooler.
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.
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.
Align the jiggle valve with the upper side of the stud bolt.
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, 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 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.
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.
July, 26, 2015 AT 8:10 AM
The water inlet housing on my 2000 Avalon has THREE (3) studs/nuts, NOT two. Hopefully this will save some people some difficulty. I know other versions do only have two. I have the 1MZFE engine version MCX20L manufactured in 03/00. The picture shown here actually shows 3 studs but the wording says 2.