Parts for your 2017 Subaru Legacy-Radiator cap

2017 Subaru Legacy radiator-cap — what it is, where it lives, and how to look after it

Yes, a radiator cap is used on the 2017 Subaru Legacy. On this model, Subaru places the pressure cap on a pressurised coolant filler/header tank (also called a surge tank) rather than directly on the radiator. Technical references include: 2017 Legacy Owner’s Manual (Cooling System section, warnings about removing the radiator cap on a hot engine and coolant level checks), Subaru Legacy/Outback 2015–2019 Service Manual (Cooling, Radiator Cap Inspection, pressure specification approx. 108 kPa/1.1 bar), and Subaru technical training materials describing the pressurised header-tank layout used on FB and EZ engines.

For this vehicle, the “radiator cap” is still the correct part, even though it’s fitted to the header tank. Its job is simple but crucial. The cap holds system pressure so coolant can run hotter without boiling, which improves heat transfer and keeps the Legacy comfy on scorching summer runs or long climbs. Inside the cap are two valves: a pressure valve that opens to bleed excess pressure to the overflow bottle, and a vacuum valve that lets coolant pull back in as things cool down. That back-and-forth keeps air out and the system topped up.

When a cap gets tired, it can cause hard-to-trace dramas: slow coolant loss, random temperature swings, weak cabin heat, hoses that feel oddly soft or collapse after shutdown, even premature water pump wear. Because it’s cheap insurance, most workshops in AU/NZ test the cap at regular services and replace it if it won’t hold spec (around 1.1 bar is typical for the 2017 Legacy).

Good practice for owners and techs:

• Only remove the cap when the engine is fully cold.
• Inspect the rubber seals for nicks, flattening, or crusty deposits, replace if suspect.
• Stick with an OEM-quality 1.1 bar cap. Mismatched ratings can trigger overheating or coolant loss.
• If renewing coolant, test or replace the cap at the same time. Use the correct long-life Subaru coolant and bleed air via the header tank as specified in the service manual.

Treating the radiator cap as a routine service item helps the 2017 Subaru Legacy avoid overheating headaches and keeps the heater, pump, and radiator working as they should.

Technical sources (no external links):

• Subaru Legacy/Outback 2015–2019 Service Manual — Cooling: Radiator Cap Inspection (pressure spec approx. 108 kPa/1.1 bar).
• 2017 Subaru Legacy Owner’s Manual — Cooling System section (cap location, hot-removal warnings).
• Subaru technical training materials on pressurised header tank cooling layouts for FB/EZ engines.

Popular questions about the 2017 Subaru Legacy radiator-cap

Does the 2017 Subaru Legacy have a cap on the radiator itself?
The system uses a pressurised header/surge tank with the cap mounted on that tank, not on the radiator neck. It’s still the “radiator cap” by name and function, just fitted in a smarter location for bleeding and consistent coolant flow.

What pressure rating should the cap be?
Subaru specifies roughly 108 kPa (1.1 bar). Using a cap with a lower rating can cause boil-over and overflow, a higher rating can stress hoses and the radiator. Stick with OEM or equivalent quality at 1.1 bar.

How often should the radiator cap be replaced?
Have it pressure-tested at routine services. Replace it if it fails to hold spec, shows damaged seals, or during major cooling system work. Many techs treat it as a preventative replacement with coolant changes for peace of mind.