Parts for your 1998 Subaru Forester-Thermostat

1998 Subaru Forester Thermostat — Purpose, Service and Replacement

Yes, the 1998 Subaru Forester is fitted with a thermostat. This is confirmed by the Subaru Factory Service Manual for the SF-series Forester (Cooling System section) and mainstream parts catalogues used by workshops across Australia and New Zealand, which list a dedicated engine thermostat and gasket for the EJ20/EJ25 engines used in this model. These technical sources specify an OEM-style thermostat that begins to open around the high‑70s Celsius and is fully open in the mid‑90s Celsius range, making it a critical, standard component of the cooling system.

On this Forester, the thermostat sits at the water pump outlet where the lower radiator hose connects. Its job is to regulate coolant flow so the engine warms up quickly, stays at a steady operating temperature, and avoids overheating. When cold, it stays shut to speed warm‑up, as coolant reaches spec temperature, it opens to send heat to the radiator. That steady temp keeps fuel economy, emissions, and engine wear in check.

For servicing, most technicians treat the thermostat as “replace when there’s a reason” rather than a strict time item. It’s a smart add‑on during timing belt and water pump jobs, or any time there are cooling complaints. Use an OEM‑equivalent thermostat with the bleed/jiggle pin oriented at the top, fit a fresh gasket, and refill with the correct Subaru‑approved ethylene glycol coolant mixed 50/50 with demineralised water. After refilling via the radiator cap (not just the overflow bottle), run the heater on hot and bleed out air, top up as the level drops and recheck after a short drive once cooled.

• Common symptoms of a dodgy thermostat: slow warm‑up or running too cool, temp gauge wandering, poor cabin heat, sudden overheating, or pressure pushing coolant into the overflow.
• Best practice: inspect hoses and the radiator cap, check for leaks around the thermostat housing, and replace the thermostat if there’s any doubt—cheap insurance for EJ engines.

If unsure on torque specs or bleeding steps, follow the Subaru service manual procedures used by dealerships and trade workshops. Done right, a quality thermostat helps the old Forester run sweet and stay reliable on Aussie and Kiwi roads.

Popular questions about the 1998 Subaru Forester thermostat

What temperature should the thermostat open?
The factory spec is to begin opening in the high‑70s Celsius (around 78–82°C) and be fully open in the mid‑90s°C range. If the gauge never reaches normal or the engine overheats before fans cycle, the thermostat could be suspect.

Where is the thermostat located?
It’s mounted at the water pump, behind the lower radiator hose outlet at the front of the engine. Remove the lower hose and the alloy outlet neck to access it. Always refit the bleed/jiggle pin at the 12 o’clock position.

How often should it be replaced?
There’s no strict interval, but it’s commonly replaced during timing belt and water pump services, or any time there are overheating/undercooling symptoms. Using an OEM‑style unit and fresh gasket is recommended for long‑term reliability.