Parts for your 2007 Subaru Legacy-Thermostat

2007 Subaru Legacy thermostat: purpose, servicing and tips

Per the Subaru Factory Service Manual (2007 Legacy/Outback BL/BP, Cooling System section) and the Subaru electronic parts catalogue for EJ and EZ engines, the 2007 Subaru Legacy is fitted with a wax‑pellet engine coolant thermostat. It sits in the water pump outlet housing where the lower radiator hose connects. So a thermostat absolutely is used on this model.

The thermostat’s job is to get the flat-four or H6 up to the right temp quickly and keep it there. It stays shut while the engine warms up, then begins opening around the specified temperature so coolant can circulate through the radiator. That steady operating temp helps fuel economy, emissions, heater performance and engine longevity. If it sticks open, the Legacy takes ages to warm up and sips more fuel, if it sticks shut, overheating can happen fast.

While Subaru doesn’t call the thermostat a routine replacement item by mileage alone, it’s smart to replace it proactively when the cooling system is opened for major work—think timing belt and water pump service on EJ models—or any time overheating or slow warm‑up shows up. Always use a quality, correct‑spec thermostat and a new seal.

• Good times to replace: during timing belt/water pump service, after an overheat, if the gauge is erratic, if the cabin heater is weak and warm‑up is slow.
• Tell‑tale symptoms:
  • Stuck open: long warm‑up, poor heater output, higher fuel use, temp gauge sitting low on the move.
  • Stuck closed: rapid overheating, hard upper hose, coolant pushing into the reservoir.

Basic replacement advice under the bonnet: work on a stone‑cold engine, drain enough coolant to drop the level below the thermostat, remove the lower hose and housing, swap the thermostat and seal in the correct orientation, refit and torque the housing to factory spec. Refill with the correct Subaru‑approved coolant at about a 50/50 mix with demineralised water. Bleed the system with the heater on full hot, squeezing the upper hose gently and letting the fans cycle. Top up the radiator and overflow bottle once cooled and recheck over the next few drives.

If the old coolant looks rusty or discoloured, consider a full flush. And if a non‑genuine thermostat is suspected, going back to a proper Subaru‑spec unit often cures flaky temp behaviour.

FAQs

Where is the thermostat on a 2007 Subaru Legacy?
It’s mounted in the water pump outlet at the front of the engine, behind the lower radiator hose connection. Access is from underneath or the front with the splash shield removed. The design is the same idea across the EJ four‑cylinders and the EZ30 H6—low and forward in the cooling circuit.

What temperature does the Legacy thermostat open at?
Subaru specifies a thermostat that begins to open in the high‑70s to low‑80s °C range and is fully open a bit above 90 °C. Using the proper spec ensures quick warm‑up without running hot under load, which keeps emissions and fuel use in check and gives strong heater performance in winter.

Should it be replaced on a schedule?
Not strictly by kilometres alone. It’s best replaced when doing the timing belt and water pump on EJ models, after any overheat, or when symptoms point to it. If the cooling system is being refreshed—hoses, pump, coolant—adding a new thermostat and seal is low‑cost insurance.