Parts for your 2009 Subaru Outback-Thermostat housing

2009 Subaru Outback thermostat housing — purpose, service tips, and when to replace

A thermostat housing is absolutely used on the 2009 Subaru Outback. Both the 2.5‑litre EJ253 four-cylinder and the 3.0‑litre EZ30 six-cylinder place the thermostat in the water pump inlet and clamp it with a two‑bolt alloy cover commonly referred to as the thermostat housing. This layout is shown in the Subaru 2009 Legacy/Outback Factory Service Manual (Cooling System – Thermostat and Water Pump sections) and the Subaru Electronic Parts Catalogue (EPC), which list the thermostat, housing/cover and sealing gasket/O‑ring as separate service parts.

On this model, the thermostat housing does more than just cap the thermostat. It’s the lower radiator hose connection, a sealing face for the water pump inlet, and the locating register that keeps the thermostat seated correctly. By holding the thermostat in place and directing coolant flow, it helps the engine warm up promptly and then stay at a stable operating temperature, which protects the head gaskets and keeps fuel economy on point.

As part of routine servicing, it’s worth having a quick look under the bonnet for tell‑tale signs around the thermostat housing: dried coolant staining, dampness at the hose neck, or a weep at the gasket line. The housing on these Subarus is cast aluminium, so corrosion or pitting can occur if incorrect coolant is used or changes are skipped. If the flange is warped or pitted, replace the housing rather than trying to seal it with goop.

When replacing the thermostat or housing on a 2009 Outback, it’s smart practice to:

• Fit a new Subaru‑spec thermostat and the correct O‑ring/gasket every time.
• Clean and de‑scale the mating faces, don’t scratch the water pump surface.
• Avoid RTV unless the workshop manual explicitly calls for it, the seal is via the O‑ring/gasket.
• Tighten the two housing bolts evenly to factory torque (around 9 N·m, always confirm in the Subaru FSM for the exact engine).
• Refill with Subaru‑approved coolant (often the blue long‑life Super Coolant on many 2009 cars), bleed air with the heater on HOT, and recheck the level after a short drive.

The housing itself isn’t a scheduled replacement item, but many owners choose to renew the thermostat and gasket during major cooling system work (e.g., water pump service) or if any leak, corrosion or temperature fluctuation shows up. Done properly, it’s a tidy, low‑cost job that can save headaches down the track.

Where is the thermostat housing on a 2009 Subaru Outback?
It sits at the front of the engine where the lower radiator hose meets the water pump. On the EJ253 four‑cylinder, look low on the passenger side (RHD) behind the lower hose — two small bolts hold the alloy cover. On the EZ30 six‑cylinder, it’s also at the front, tucked near the lower hose connection. A torch helps spot the bolt heads and gasket line.

Do you need sealant on the housing gasket?
No. Subaru uses a formed gasket or O‑ring that seals dry on clean surfaces. A light dab only to hold the gasket in place is fine, but smearing RTV around the flange can cause leaks or debris in the cooling system. Always follow the Subaru workshop manual if a specific sealant is called for.

What torque should the thermostat housing bolts be tightened to?
They’re small M6 bolts and only need light torque — about 9 N·m is typical on both EJ25 and EZ30 engines. The key is tightening evenly and not over‑doing it. If in doubt, check the Subaru Factory Service Manual for your exact VIN and engine code.