Parts for your 2018 Subaru Legacy-Thermostat

2018 Subaru Legacy thermostat — what it does, and when to sort it out

Yes, a thermostat is absolutely fitted to the 2018 Subaru Legacy. The Subaru Factory Service Manual (Cooling System section) for the 2018 Legacy and Subaru parts catalogues specify a conventional engine coolant thermostat mounted in the water pump inlet housing on FB-series four-cylinder and EZ-series six-cylinder engines. The FSM cites an opening temperature around 88°C and full opening near 100°C, confirming it’s a standard, serviceable component.

This little valve has a big job. It keeps the engine at the sweet-spot temperature by staying shut while the engine warms up, then opening to let coolant flow through the radiator once it’s hot enough. That helps fuel economy, reduces wear, and keeps cabin heat consistent. On a 2018 Legacy, a healthy thermostat means faster warm-ups on a cold morning and stable temps when towing, cruising, or stuck in traffic under the Aussie or Kiwi sun.

Thermostats aren’t a scheduled “replace every X km” item on these cars, they’re replaced on condition. Still, they’re worth a look any time the coolant is changed or if there are temperature complaints. Common red flags include:

• Slow warm-up or the temp gauge sitting low (stuck open)
• Overheating or boiling into the overflow (stuck closed)
• Heater goes cold at speed, hot at idle, or temperature swings
• Upper hose staying cool while the engine’s clearly hot

When replacement’s due, it pays to use a genuine Subaru thermostat and new O-ring/gasket. Orientation matters — the jiggle pin/air bleed should face up as per the FSM — and the housing fasteners must be torqued to spec. On refill, use the correct Subaru long-life coolant, don’t mix types, and bleed air thoroughly, these flat engines can trap air if rushed. After the job, confirm fan operation, check for leaks, and recheck the coolant level under the bonnet after a couple of heat cycles.

For owners who like to DIY, a quality funnel, fresh clamps if the old ones are tired, and patience during bleeding go a long way. For everyone else, any competent workshop can test and swap a thermostat quickly, often alongside a coolant service for tidy value.

What temperature does the 2018 Subaru Legacy thermostat open at?

Per the Subaru Factory Service Manual, the thermostat begins to open at roughly 88°C and is fully open close to 100°C. That range keeps the engine in its ideal operating window for performance and longevity.

How can someone tell if the thermostat is failing on a 2018 Legacy?

Watch for overheating, a temp gauge that never reaches normal, heater output that’s inconsistent, or hoses that stay cool when they shouldn’t. A workshop can confirm with an infrared thermometer, scan tool data, and a hot/cold hose check.

Is it safe to drive with a dodgy thermostat?

Not really. A stuck-closed thermostat can cause rapid overheating and engine damage. A stuck-open unit can wash fuel economy, cause poor heater performance, and increase engine wear. Best to address it promptly.