Parts for your 2009 Subaru Legacy-Thermostat housing

2009 Subaru Legacy (Liberty) Thermostat Housing — What It Does and How to Look After It

Yes, the 2009 Subaru Legacy/Liberty uses a thermostat housing. Subaru’s Factory Service Manual for the 2009 model (Engine Cooling section) specifies the thermostat mounted in a bolt-on water inlet—commonly referred to as the thermostat housing—at the lower radiator hose on the front of the EJ25 engine. Subaru’s FAST electronic parts catalogue lists a dedicated water inlet/thermostat case and seal for 2009 EJ253/EJ255 variants, and major technical catalogues from Gates and Dayco also supply housings for this application. So the thermostat housing is relevant and fitted to this vehicle.

On the 2009 Legacy, the thermostat housing secures the thermostat and forms the passage where coolant returns from the radiator to the water pump. Because Subaru locates the thermostat on the suction side of the pump (lower hose), it accurately regulates engine temperature by metering cooled radiator flow back into the engine. The housing also provides the sealing surface for the thermostat’s O-ring, keeping the system watertight under pressure.

As part of routine servicing, it’s smart to inspect the housing whenever coolant is changed or the lower hose is off. Look for corrosion, pitting where the O-ring seats, hairline cracks, or warping from past overheating. Any of these can cause weeps or sudden leaks. If replacing the thermostat, fit a quality OEM-spec unit and new O-ring, clean the mating surfaces gently with a plastic scraper and a lint-free cloth, and avoid sealants unless the service manual explicitly calls for them.

Replacement is straightforward: let the engine cool completely, drain enough coolant, remove the lower radiator hose, undo the two housing bolts, and lift the cover. Note thermostat orientation—on EJ engines the jiggle pin/air bleed faces up—then seat the new O-ring evenly. Refit the housing, reconnect the hose, and refill with the correct coolant (Subaru long-life blue or an equivalent phosphate OAT formula) mixed 50/50 with demineralised water if not pre-mixed. Bleed air by parking nose-up, turning the heater to hot, idling with the cap off until the fans cycle, and topping up as bubbles purge. Recheck levels and inspect for leaks after a short drive.

Common clues the housing or seal needs attention include a sweet coolant smell under the bonnet, dried pink/white residue around the lower hose flange, slow warm-up (stuck-open thermostat), or overheating (stuck-closed). Given low part cost and high consequences of leaks or temperature swings, renewing the housing and hardware when doing a thermostat is a sensible bit of preventative maintenance for Aussie and Kiwi owners clocking up the kilometres.

Does a 2009 Subaru Legacy have a thermostat housing, and where is it?

It does. On EJ25-equipped 2009 models, the thermostat sits in a bolt-on water inlet (thermostat housing) at the lower radiator hose, mounted to the front of the engine’s water pump. You’ll find it low and forward, just behind the front bumper line.

What coolant should be used after replacing the housing or thermostat?

Use Subaru long-life blue coolant or an equivalent phosphate OAT formula, typically a 50/50 premix. Avoid heavily silicated coolants. After refilling, bleed the system properly so no air pockets remain—run the heater on hot, idle until fans cycle, and top up as bubbles purge.

Should the housing be replaced with the thermostat, or only if it’s damaged?

If the housing shows corrosion, pitting at the seal land, or any distortion, replace it. Many techs renew the housing, O-ring, and thermostat together to avoid repeat labour and ensure a perfect seal, especially on higher-kilometre vehicles.