2026-10-08 · 5 min read
Subaru WRX STI Thermostat Replacement Malaysia
The EJ257 thermostat regulates engine temperature — a faulty thermostat that sticks open causes the engine to run chronically cool (reducing efficiency), while one that sticks closed causes dangerous overheating that can trigger head gasket failure. Inspect the thermostat when doing a coolant flush or timing belt service. OEM thermostat replacement is recommended; aftermarket units should meet original temperature specifications.
Subaru WRX STI Thermostat
- STI EJ257 thermostat: opens at approximately 71°C — deliberately lower than NA Subaru; EJ257 high-compression high-boost engine needs to stay cooler; lower thermostat spec is correct for EJ257 — do not replace with FB25 thermostat
- STI thermostat housing: aluminium housing on EJ257; inspect for corrosion and seepage at 80,000 km; old coolant causes aluminium corrosion at housing — another reason to change coolant every 80,000 km on STI
- STI thermostat and knock: if EJ257 thermostat fails open and engine runs cooler than 71°C — knock threshold changes; ECU expects warm engine for timing maps; cold engine on STI can trigger timing retard and reduce power
- STI thermostat replacement: fit OEM or quality thermostat rated for EJ257 spec (71°C); do not use generic thermostat — incorrect opening temp affects engine management; always bleed cooling system after thermostat replacement