Parts for your 2008 Subaru Impreza-Thermostat housing

2008 Subaru Impreza Thermostat Housing

Technical references including the Subaru Factory Service Manual for the 2008 Impreza (GE/GH/GR platforms) and Subaru’s genuine parts catalogue confirm the model uses a thermostat secured by a bolt-on cover at the water pump inlet—commonly referred to as the thermostat housing. This applies to the 1.5L EL15 and the 2.5L EJ-series engines (EJ253/EJ255/EJ257), so the thermostat housing is very much a relevant, serviceable part on a 2008 Subaru Impreza.

The thermostat housing’s job is straightforward but critical. It clamps the thermostat in place, seals the coolant passage with an O-ring or gasket, and directs coolant from the lower radiator hose into the water pump. When everything’s healthy, the engine warms up quickly, then holds a steady operating temperature—key for performance, economy, and long engine life.

On a 2008 Impreza, the housing sits low at the front of the engine, right where the lower radiator hose meets the water pump. Over time, the sealing surface can pit, the O-ring can flatten or harden, and the mating bolts can be over-torqued or corrode. Any of these can lead to small leaks, slow temperature rise, or overheating headaches on a hot Kiwi or Aussie day in traffic.

During servicing, it’s smart to inspect the housing whenever the coolant is changed or the thermostat is replaced. Look for white or green crusty residue, staining around the bolt heads, or dampness near the lower hose. If the housing face is corroded or warped, replace it rather than trying to “make do” with sealant. Always fit a new thermostat gasket/O-ring, clean the mating surfaces, and tighten the bolts evenly to the light torque specified in the Subaru workshop manual.

When refilling, use Subaru-approved long-life coolant (silicate-free, phosphate organic-acid technology) at the correct 50/50 mix with demineralised water unless pre-mixed. Bleed the system carefully: set the heater to hot, run the engine until the fans cycle, squeeze the upper and lower hoses to purge air, and top up after a full cool-down. Recheck for leaks over the next few drives.

If the car shows fluctuating temps, slow warm-up, or a drip at the front underside, a new thermostat and a tidy housing reseal can restore proper cooling. It’s a simple, driveway-friendly job with a jack, stands, basic spanners, and patience.

• Replace the thermostat and O-ring with quality parts.
• Inspect/replace the housing if pitted, cracked, or warped.
• Use the correct coolant and bleed air thoroughly.

Popular questions about the 2008 Subaru Impreza thermostat housing

Where is the thermostat housing located on a 2008 Impreza?
It’s mounted low at the front of the engine, forming the inlet to the water pump. The lower radiator hose connects directly to it, so it’s easy to spot once the splash shield is off.

Do you need to replace the housing when changing the thermostat?
Not always. If the housing’s sealing face is clean and flat, and there are no cracks or corrosion, just fit a new thermostat and O-ring. Replace the housing if there’s pitting, warping, or a past leak that’s etched the surface.

Should sealant be used on the thermostat housing?
Generally no. Subaru designs this joint to seal with a dedicated O-ring or gasket. Clean surfaces and correct torque are the go. Only use sealant if a specific repair instruction in the workshop manual calls for it.