Parts for your 2000 Subaru Legacy-Thermostat housing

2000 Subaru Legacy thermostat housing — purpose, care, and when to replace

Yes, a thermostat housing is absolutely used on the 2000 Subaru Legacy. Subaru’s Factory Service Manual for the 2000 Legacy (Cooling System section) and the genuine Subaru parts catalogue (Group 21 – Water Pump/Thermostat) both show a water inlet/thermostat cover bolted to the water pump. That alloy housing secures the thermostat and seals it with an O-ring where the lower radiator hose meets the pump. Haynes/Chilton workshop manuals for EJ engines describe the same setup.

On this model, the thermostat housing’s job is to hold the thermostat in the correct orientation, seal the coolant passage, and direct flow from the lower radiator hose into the water pump. When the engine’s cold, the thermostat stays shut, helping it warm up quickly. Once it hits operating temperature, the thermostat opens and the housing channels coolant to keep temps steady under all sorts of Kiwi and Aussie driving conditions.

If the housing or its O-ring isn’t right, cooling drama follows. Common giveaways include:

• Coolant weeping around the lower radiator hose flange
• Overheating at highway speeds or under load
• Slow warm-up or temp gauge wandering
• Dried crusty residue around the housing bolts

Servicing is straight-up if done carefully. Always start with a dead-cool engine. Drain enough coolant to drop the level below the housing. Remove the lower hose, undo the two housing bolts, and lift the cover away. Fit a quality thermostat with the bleed pin/jiggle valve at 12 o’clock, install a new O-ring, and refit the housing. Don’t use RTV on the O-ring, the factory design seals dry. Tighten bolts evenly to the Subaru spec in the service manual. Reconnect the hose, refill with the correct Subaru-compatible long-life coolant mix, and bleed the system thoroughly.

For ongoing care, change coolant at the recommended interval to minimise corrosion and pitting in the alloy housing. Any time the housing comes off, replace the O-ring and inspect the hose neck for scoring or distortion. If the cover’s badly pitted or warped, replace it—parts are inexpensive compared with the hassle of repeat leaks. A quick look under the bonnet for dampness or staining around the water pump inlet at regular services can save heaps of time later.

Popular questions

Where is the thermostat housing on a 2000 Subaru Legacy?
It sits at the front of the engine where the lower radiator hose connects to the water pump. From under the bonnet, look low and centre-front, the alloy elbow secured by two bolts is the housing.

Do I need sealant on the thermostat housing?
No sealant is used on the O-ring in this application. Fit a new O-ring, ensure both mating surfaces are clean, and tighten the housing evenly to factory torque. RTV or gasket goo can cause leaks or misalignment.

How often should the thermostat or housing be replaced?
There’s no fixed kilometre count for the housing—replace it only if corroded or damaged. The thermostat is typically replaced when there are cooling issues or during major cooling system work. Always renew the O-ring whenever the housing is removed.