Parts for your 2015 Subaru Legacy-Radiator cap

2015 Subaru Legacy radiator cap – what it does, where it lives, and when to swap it

Based on factory literature, the 2015 Subaru Legacy does use a radiator cap, though it’s mounted on the pressurised coolant filler tank (header/degas bottle) rather than on the radiator itself. Subaru’s 2015 Legacy/Outback Service Manual (Cooling: Radiator Cap Inspection) specifies the cap’s relief pressure at approximately 108–137 kPa, and the Owner’s Manual cautions against opening the coolant filler tank cap when hot—both confirming the presence and function of a pressure cap. Subaru parts catalogues for MY2015 Legacy also list a pressure cap for the cooling system’s filler tank assembly.

On this model, the “radiator cap” is the pressure cap on the highest point of the system—the coolant filler tank near the top of the engine—so the radiator core itself doesn’t need a separate cap. That cap manages system pressure, raises the coolant’s boiling point, and allows coolant to move between the tank and overflow as temperatures change.

What’s the cap actually doing? Three big jobs:

• It seals the system so pressure can build, lifting the coolant’s boiling point and keeping temps stable on hot days and steep climbs.
• It relieves excess pressure if it spikes, protecting hoses, the radiator and the heater core.
• It enables coolant recovery as the engine heats and cools, pulling fluid back in so air doesn’t get trapped.

Good servicing on a 2015 Legacy should include a quick cap check under the bonnet. A tired cap can cause slow overheating, low heater performance, random coolant loss, or hard upper hoses after shutdown. If the rubber seals are cracked, the spring feels weak, or the seat is crusty with deposits, it’s time for a new one.

1. Inspect the cap every service (or at least each 10,000–15,000 km): look for perished seals, bent tangs, gunk on the sealing face and a lazy spring.
2. Only open when stone cold. Use a rag and twist slowly to relieve any residual pressure.
3. Match the pressure rating to spec (around 108–137 kPa for this model). Using the wrong rating can cause boil-over or stress the system.
4. If in doubt, replace preventatively—many techs swap caps every 4–5 years or when doing a coolant service.
5. Top up with the correct Subaru long-life coolant mix and bleed air per the workshop procedure to avoid hot spots.

Genuine or high-quality equivalent caps are inexpensive insurance, especially with the Legacy’s pressurised filler-tank layout common across modern Subarus.

Popular questions

Does the 2015 Subaru Legacy actually have a radiator cap?

Yes. It’s a pressure cap, but it sits on the coolant filler tank (header/degas bottle) rather than on the radiator. It performs the same job—controlling pressure and coolant recovery.

What pressure rating cap should be used?

Subaru service information lists an opening pressure in the ballpark of 108–137 kPa for this model year. Always match the vehicle spec sticker, the Owner’s Manual, or a trusted parts catalogue for the exact rating.

How often should the cap be replaced?

Inspect it at every service and replace if seals are cracked, the spring is weak, or there are signs of leakage. Many workshops proactively replace the cap every 4–5 years or at the time of a coolant change for peace of mind.