Parts for your 1999 Subaru Forester-Thermostat housing

1999 Subaru Forester Thermostat Housing — what it does and how to keep it happy

Technical references confirm the 1999 Subaru Forester is fitted with a thermostat housing. The Subaru Factory Service Manual (Forester SF, Cooling System) shows the thermostat seated in the water pump outlet and secured by a dedicated cover/housing. The Subaru FAST electronic parts catalogue for MY99 Forester diagrams the same “thermostat cover/water outlet” at the lower radiator hose connection. Independent application guides (e.g., Gates and Dayco Australia/NZ) also list the thermostat as installed in the engine’s outlet with a sealing ring and cover, not a remote inline unit.

On the ’99 Forester, the thermostat housing does more than just cap the thermostat. It locates and clamps the thermostat in the water pump outlet, seals coolant with an O-ring or gasket, and directs flow into the lower radiator hose. By keeping the thermostat properly seated and leak-free, it helps the EJ-series engine reach and hold the right operating temperature, stabilising cabin heat, fuel economy, and engine longevity.

When serviced, the housing is typically reused unless it’s corroded, pitted, or warped. Forester owners often notice housing-related issues as coolant seepage at the lower hose outlet, temperature gauge swings, or slow warm-up. Any crusty aluminium, distorted mating faces, or damaged bolt threads are cues to replace the cover along with the thermostat and seal.

Best practice during replacement is straightforward and workshop-friendly:

• Drain enough coolant to drop the level below the housing, remove the lower hose and the two housing bolts.
• Clean the mating surfaces carefully, install a quality OEM-spec thermostat with a fresh O-ring/gasket (no extra sealant unless the FSM explicitly allows it).
• Orient the jiggle pin/air bleed to the top as specified, refit the cover, and tighten bolts evenly to the factory torque.
• Refill with the correct Subaru-approved coolant mix, run the heater on hot, and bleed air patiently to prevent hot spots.

As part of routine servicing, checking the housing for weeps and renewing coolant at the interval appropriate to the fluid type (conventional or long-life) keeps the system sweet. Sticking with genuine-spec thermostats and seals pays off on EJ engines, helping avoid overcooling, overheating, or nuisance leaks. For high-kilometre cars, a quick look at the housing any time the lower hose is off is cheap insurance, and replacing a tired cover during a thermostat or water pump job is a sensible call in Australian and New Zealand conditions.

Popular questions

Where is the thermostat housing on a 1999 Forester?
It’s mounted at the bottom front of the engine, integrated with the water pump outlet where the lower radiator hose connects. The cover is held on with two bolts, remove the hose and cover to access the thermostat.

Does the housing need replacing or just the thermostat?
Usually only the thermostat and its O-ring/gasket are replaced. The housing is reused unless it’s corroded, pitted, cracked, or warped. If the sealing face isn’t perfect, swapping the cover avoids repeat leaks.

Should sealant be used on the housing?
The Forester’s thermostat uses a dedicated O-ring or gasket, so additional sealant is generally not required. Follow the Factory Service Manual, fit a new seal, orient the thermostat correctly, and tighten the bolts to spec.