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