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