Parts for your 2009 Subaru Forester-Radiator cap

2009 Subaru Forester radiator cap — what it does and when to replace it

Technical sources confirm a pressure-type radiator cap is used on the 2009 Subaru Forester. The Subaru Forester (SH, 2009) Factory Service Manual cooling-system section specifies a system pressure cap (approx. 108 kPa/1.1 bar), and Subaru’s electronic parts catalogue lists a serviceable pressure cap for this model. On non‑turbo 2.5X (EJ253) models the cap sits on the radiator neck, on turbo XT (EJ255) models the pressure cap is fitted to the pressurised header tank under the bonnet rather than on the radiator itself. Different location, same job.

On this Forester, the radiator cap is a small but critical bit of kit. It seals the cooling system and holds pressure so coolant runs hotter without boiling, keeping temps stable on long Kiwi and Aussie drives. Inside the cap are two valves: a pressure relief that lets excess pressure and coolant move to the overflow when things heat up, and a vacuum valve that draws coolant back in as the engine cools. That push‑pull keeps hoses from collapsing, prevents air from sneaking in, and helps the system self‑recover after a hot run.

If the cap’s spring weakens or the rubber seals go hard, they’ll see tell‑tales like rising temps at highway speed, coolant dribbling out of the overflow bottle, sweet smells, or hoses that collapse after shut‑down. Because it’s cheap insurance, the cap deserves routine attention during a cooling‑system service on any 2009 Forester.

What’s recommended? Stick with the correct pressure rating (about 108 kPa as specified in Subaru service literature) and an OE‑quality cap. Inspect it at each service: look for cracked rubber, perished seals, corroded seats, or gunk on the filler neck. A workshop can pressure‑test the cap, if it doesn’t hold spec, replace it. As a rule of thumb, many owners refresh the cap every 4–5 years or 80,000–100,000 kilometres, earlier if there are symptoms.

• Only remove the cap stone‑cold, never when hot or even warm.
• Check and clean the filler neck so the seal sits square.
• Top up with Subaru‑approved coolant mix and bleed air as per the service manual.
• Turbo XT owners: remember the pressure cap lives on the header tank, the radiator may have no cap by design.

References: Subaru Forester (SH, 2009) Factory Service Manual – Engine/Cooling System, Subaru Electronic Parts Catalogue for 2009 Forester applications, common aftermarket catalogues matching an approx. 1.1 bar cap to SH Forester.

Popular questions

Where is the radiator cap on a 2009 Subaru Forester?
Non‑turbo 2.5X models have the cap on the radiator filler neck. Turbo XT models use a pressurised header tank with the cap on that tank, and the radiator itself typically has no cap. If unsure, look near the upper radiator hose for the filler neck (NA) or for the alloy/plastic header tank (XT) under the bonnet.

What pressure rating should the cap be?
Subaru service information calls for a pressure cap around 108 kPa (about 1.1 bar). Matching the rating printed on the original cap is the safest bet. A cap that’s too low can cause boiling and overflow, too high can stress hoses, the radiator, and the heater core.

How often should the radiator cap be replaced?
Have it checked at every service. Many owners replace the cap every 4–5 years or 80,000–100,000 km, or immediately if there are signs of leakage, hardened seals, or pressure‑test failure. It’s a low‑cost part that protects much pricier components.