Parts for your 2009 Subaru Impreza-Thermostat housing

2009 Subaru Impreza Thermostat Housing: What It Does and When To Service It

Yes, the 2009 Subaru Impreza uses a thermostat housing. Factory service manuals for the 2008–2011 Impreza range (EJ-series engines) and OEM parts catalogues list a bolt-on thermostat cover/water inlet mounted on the front of the engine at the water pump, where the lower radiator hose connects. That assembly retains the thermostat, seals the coolant circuit, and directs flow back into the engine.

On the 2009 Impreza, the thermostat housing’s job is to clamp the thermostat in place, seal coolant with an O-ring, and channel return flow from the radiator into the water pump. Because the EJ engine regulates temperature from the lower hose side, a sound housing is critical to stable warm-up and consistent operating temps. If the housing face is pitted or the O-ring is flattened, you’ll often see weeping at the lower hose area, temperature swings, or slow warm-up.

During routine servicing, it’s smart to inspect the housing any time the coolant is changed or the thermostat is out. Look for corrosion, warping, or thread damage, and always replace the O-ring/gasket. Many owners time thermostat and housing checks with a timing-belt/water-pump job (around 160,000 km, depending on market guidance), as the cooling system is already open. The housing on these Subarus is metal, but age and poor coolant can still cause pitting on the sealing face—if it won’t seal cleanly, replace it.

Replacement tips for the home mechanic under the bonnet:

• Use the correct thermostat and a fresh O-ring, lightly clean mating surfaces—no gouging.
• Install the thermostat with any jiggle pin/air bleed at the top, then refit the housing with light, even torque to factory spec—don’t overdo it.
• Refill with quality, phosphate non-silicate long-life coolant (many use Subaru’s blue premix) at 50/50. Bleed air with the heater on hot, nose slightly up, and wait for the fans to cycle. Top up the overflow bottle after a short drive.

Watch for clues that the housing or thermostat needs attention: coolant smell or drips near the lower radiator hose, random temp gauge movement, overheating on climbs, or a stubbornly cold cabin due to air pockets. Keeping the housing clean, sealed, and torqued correctly helps the EJ engine stay happy across Aussie and Kiwi conditions, from city traffic to long open-road stints.

FAQs

Where is the thermostat housing on a 2009 Subaru Impreza?
It’s at the front of the engine, low down on the water pump where the lower radiator hose joins. From the front of the car, look under the radiator line—two small bolts secure the housing (thermostat cover) to the pump.

Do you need to replace the housing when changing the thermostat?
Not always. If the housing face is clean and flat and the bolt threads are good, it can be reused with a new O-ring. Replace the housing if there’s pitting, warping, or damage that prevents a reliable seal.

What coolant should be used, and how is air bled out?
Use a high-quality, long-life coolant suitable for Subaru aluminium engines (many choose Subaru Super Coolant). Refill 50/50, run the engine with the heater on hot, nose slightly elevated, and wait for the fans to cycle while topping up as bubbles escape. Recheck levels after a short drive.