Parts for your 2018 Subaru Forester-Radiator cap

2018 Subaru Forester radiator cap — what it does and how to look after it

Based on Subaru’s technical material, the 2018 Forester does use a radiator cap and it’s a relevant service item. The 2018 Forester (SJ series) Owner’s Manual warns against opening the radiator cap when hot, and the Subaru Service Manual for the SJ platform lists a radiator cap pressure specification of approximately 108 kPa (1.1 bar) with a test procedure for sealing and pressure retention. Genuine parts catalogues also show a dedicated radiator cap (common Subaru p/n 45137AE003) for 2014–2018 Forester. That confirms the vehicle is fitted with a pressure cap rather than a cap‑less, fully sealed system.

On this Forester, the radiator cap is a small but crucial bit of kit. It seals the cooling system and holds pressure so coolant boils at a higher temperature, which helps keep the FB25 engine happy on hot days, steep climbs, or towing. The cap’s internal valves also let expanding coolant move to the overflow bottle and draw back in as things cool, maintaining a stable coolant level.

When the cap gets tired—rubber perishes, spring weakens, or the seal gets crusty—you can see slow coolant loss, hard or collapsed hoses, overheating at low speeds, or coolant stains around the neck. Because those symptoms mimic bigger issues, a simple cap swap can save a lot of head‑scratching.

For servicing, treat the radiator cap as a low‑cost preventative replacement. There’s no strict kilometre interval from Subaru, but annual inspection is smart, and many workshops replace the cap around 5–7 years or if a pressure test shows it won’t hold about 108 kPa. Always match the original pressure rating—1.1 bar (approx. 108 kPa)—and stick with quality or genuine parts.

• Only remove the cap when the engine is stone cold. Use a rag and pause at the first click to vent any residual pressure.
• Inspect the rubber seal and spring, look for nicks, flattening, or rust. Clean the filler neck before refitting.
• Top up with the correct Subaru coolant mix after cap replacement, then check levels again after a short drive.

Under the bonnet, the cap sits on the radiator neck near the upper hose. If it’s been a while since it was changed—or you’re chasing small coolant losses—refreshing the cap is an easy, affordable win for cooling reliability.

Popular question: What pressure rating is correct for a 2018 Forester radiator cap?

The factory spec is about 108 kPa (1.1 bar). Using a lower or higher rating can upset boiling protection or stress hoses, so stick with the OEM rating when replacing.

Popular question: How often should the radiator cap be replaced?

Have it checked yearly and replace it if it fails a pressure test, shows damaged seals, or every 5–7 years as cheap insurance—especially if you drive in hot climates, tow, or do long hill climbs.

Popular question: Where is the radiator cap on a 2018 Forester?

It’s on the radiator’s filler neck under the bonnet, near the upper radiator hose. Look for the metal cap with a warning label. Only open it when the engine is completely cold.