Parts for your 2018 Subaru Legacy-Radiator cap

2018 Subaru Legacy radiator cap: what it does, where it lives, and when to change it

Technical references including the Subaru Factory Service Manual for the BN/BS series (Cooling section), the 2018 Legacy Owner’s Manual, and Subaru’s parts catalogue all confirm the 2018 Subaru Legacy does use a radiator cap. It’s a pressurised cap fitted to the coolant filler tank (often called the header tank) under the bonnet, rather than on the radiator core itself. So while the radiator top tank has no cap, the “radiator cap” is absolutely relevant to this model.

On the 2018 Legacy, that cap is a small but crucial bit of kit. It seals and pressurises the cooling system, lifting the coolant’s boiling point so the engine can run at the right temperature without localised boiling. The cap also manages coolant movement between the header tank and the overflow bottle as things heat up and cool down, thanks to a pair of valves inside the cap body. When it’s doing its job properly, you’ll get stable temps, consistent cabin heat, and long water pump and hose life.

As part of routine servicing, it’s smart to treat the radiator cap like any other wear item. Under Aussie and Kiwi conditions, have it inspected at every service and replaced if there’s any doubt. A genuine or OEM-spec cap with the correct pressure rating for your engine is the go, the rating is stamped on the cap and matched in the Subaru parts listings for your VIN. If you’re unsure, ask your technician to verify the spec rather than guessing.

• Tell-tale signs a cap is due: crusty or perished rubber seals, weak spring tension, staining around the filler neck, coolant smell after shut-down, random temp swings, or the system failing a pressure test.
• Good habits: only remove the cap when the engine is stone cold, wipe the filler neck clean before refitting, replace the cap any time the cooling system is overhauled, a hose bursts, or the radiator is replaced.
• Service interval guide: inspect every service, pressure-test annually, replace every 5–7 years or 80,000–100,000 km, sooner if testing or symptoms warrant.

A fresh, correctly rated cap costs peanuts compared with the price of head gaskets or a cooked engine. Keeping it in top nick is simple insurance for your 2018 Subaru Legacy.

Popular questions

Where is the radiator cap on a 2018 Subaru Legacy?
It sits on the pressurised coolant filler (header) tank under the bonnet, typically at the top-front of the engine bay. The radiator itself doesn’t have a cap on this model.

What pressure rating cap should a 2018 Legacy use?
Use an OEM-spec Subaru cap that matches your engine and VIN. Most 2.5-litre cars use a cap around 1.1 bar (108 kPa), while other variants can differ. Check the stamping on your existing cap or your Subaru parts listing.

How often should the radiator cap be replaced?
Have it checked every service and pressure-tested yearly. As a rule of thumb, replace it every 5–7 years or 80,000–100,000 km, or immediately if it fails testing or shows damage.