Parts for your 2009 Subaru Forester-Thermostat housing

2009 Subaru Forester thermostat housing — fitment, purpose and service advice

Yes, a thermostat housing is used on the 2009 Subaru Forester. Technical documentation including the Subaru Factory Service Manual for the 2009 Forester (SH, Cooling System section) and the Subaru Electronic Parts Catalogue identify a bolt-on thermostat housing (often called the “water inlet” or “thermostat cover”) mounted to the water pump on EJ25 engines (EJ253 non-turbo and EJ255 turbo). It sits at the lower radiator hose connection on the front of the engine and clamps the thermostat in place with a sealing gasket or O-ring.

On this Forester, the thermostat housing’s job is straightforward but critical. It positions and seals the thermostat, directs coolant flow from the lower radiator hose into the water pump, and provides a leak-free junction that copes with constant heat cycles and vibration. Being a cast metal piece on these EJ engines, it’s built to last, but it’s still dependent on a healthy gasket and clean mating surfaces to keep the cooling system tight.

As part of routine servicing, it’s good practice to inspect the housing whenever coolant is changed or a thermostat is replaced. Most owners will touch this area every 5 years or around 100,000 km when coolant is renewed. A fresh gasket or O-ring should always be fitted with a new thermostat, and the housing faces should be cleaned carefully without gouging the alloy. The bolts are small, so they should be tightened evenly to the factory spec—no overdoing it with a long spanner.

Signs the housing or its seal needs attention include:

• Coolant weeping around the lower radiator hose flange or bolt heads
• Dry crust or staining (pink/green/blue) at the housing seam
• Overheating or slow warm-up paired with evidence of leaks

When replacing, stick with an OEM-spec thermostat and gasket, and check the housing for pitting or warpage, a flat edge or straightedge test under a bright light works well. If the surface isn’t true, replacing the housing is cheap insurance against repeat leaks. After refit, refill with Subaru-approved coolant (mixed correctly with demineralised water if not pre-mixed), bleed air thoroughly, and confirm steady heater output and stable operating temperature on a proper road test.

Popular questions about the 2009 Subaru Forester thermostat housing

Where is the thermostat housing on a 2009 Forester?
It’s at the front of the engine, low down at the water pump, where the lower radiator hose connects. On the EJ25, the housing bolts directly to the pump and captures the thermostat with an O-ring or gasket. Access is usually from underneath after removing the splash guard.

When should the thermostat housing be replaced?
Replace it if the mating surface is pitted or warped, if there’s persistent leakage even with a new gasket, or if corrosion has thinned the casting. Otherwise, it’s commonly reused during thermostat changes, provided it’s flat and clean. Always install a new seal and tighten to spec.

What coolant should be used after housing or thermostat work?
Use Subaru-approved long-life coolant suitable for aluminium engines. Many owners choose Subaru Super Coolant (blue) pre-mix. If using concentrate, mix to 50/50 with demineralised water. After filling, bleed air carefully and confirm the coolant level after a couple of heat cycles.