Parts for your 2008 Subaru Forester-Thermostat

2008 Subaru Forester Thermostat – purpose, servicing and smart replacement tips

Per Subaru’s Factory Service Manual for the 2008 Forester (Engine Cooling section), the official Subaru parts catalogue, and independent repair guides such as Haynes, this model absolutely uses a conventional wax‑pellet engine thermostat. It’s fitted at the water pump outlet where the lower radiator hose meets the engine, controlling coolant flow as the EJ25 warms up.

The thermostat’s job is straightforward but critical. It helps the 2.5‑litre boxer reach operating temperature quickly, then holds it steady for efficient combustion, good heater performance, and stable emissions. When cold, the thermostat stays shut to speed warm‑up. As coolant reaches roughly the high‑70s to low‑80s °C, it opens to let coolant circulate through the radiator. A healthy thermostat keeps the temperature gauge settled near the middle, even on a stinkin’ hot day or a long Kiwi hill climb.

For servicing, most workshops treat the thermostat as a “replace on condition” item, or they renew it proactively during major cooling work. On a 2008 Forester with a timing belt and common water pump service around 100,000–150,000 km, it’s sensible to fit a genuine‑spec thermostat and new gasket at the same time. Fresh coolant that meets Subaru specifications (mixed correctly) and careful bleeding to purge air are must‑dos after any cooling system work.

Typical hints the thermostat is getting tired include:

• Slow warm‑up, poor cabin heat, or a P0128 code (coolant below regulating temperature)
• Overheating at highway speeds or erratic temperature swings
• Coolant leaks or crusting around the housing

When replacing, the housing sits under the bonnet at the lower radiator hose on the engine’s front. A proper drain and refill, new O‑ring/gasket, clean mating surfaces, and correct bolt torque per the service manual keep things drama‑free. Sticking with an OEM‑type thermostat matters on Subarus, the correct jiggle‑valve/bypass design helps the boxer bleed air and maintain stable temps. After refilling, run the engine with the heater on, top up as the fans cycle, and recheck the level after a short drive. A little preventative attention here can prevent big headaches later.

Popular questions

Where is the thermostat on a 2008 Subaru Forester?
It’s mounted in the water pump outlet at the end of the lower radiator hose. Two housing bolts secure it, the sealing gasket/O‑ring should be renewed whenever it’s removed.

What temperature does the Forester thermostat open?
Factory‑spec units typically begin to open in the high‑70s to low‑80s °C range, helping the gauge sit just under halfway during normal driving. Fan operation is managed separately by the ECU and coolant temp sensor.

Should a genuine thermostat be used?
Yes, an OEM‑spec thermostat is recommended. Subaru designs rely on the correct bypass and jiggle‑valve style for consistent warm‑up and proper bleeding, which helps avoid temperature fluctuations and nuisance fault codes.