Parts for your 2018 Subaru Impreza-Radiator cap

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

Yes, a radiator cap is relevant to the 2018 Subaru Impreza. On this model (FB20 engine), the pressure cap isn’t on the radiator neck, it’s fitted to the engine-mounted coolant filler/degas tank. Subaru’s 2018 Impreza Owner’s Manual describes the cap and warns against opening it when hot, and the factory Service Manual lists “radiator cap” pressure testing with a nominal rating around 108 kPa (about 1.1 bar). Different spot, same job.

The cap is small but vital. It seals the cooling system, holds pressure so the coolant’s boiling point rises, and uses two valves: a pressure relief to vent excess pressure into the overflow bottle, and a vacuum valve to pull coolant back as the engine cools. A healthy cap helps stabilise temps, prevents localised boiling, and keeps hoses and the heater core happier.

On a 2018 Impreza, tell-tales of a dodgy cap include coolant pushing into the overflow bottle and not returning, a collapsed upper hose after cool-down, slow warm-up followed by a temp spike, or that sweet coolant whiff under the bonnet. A cap that can’t hold pressure can mimic head gasket dramas, so it’s worth sorting.

• Service check: With the engine stone cold, inspect the cap every service for cracked or flattened rubber, corrosion on the sealing faces, weak spring action, or stained crust around the neck.
• Correct spec: Use the Subaru-specified pressure rating (typically 108 kPa). The rating is stamped on the cap—don’t “upgrade” pressure without a technical reason.
• Clean fitment: Wipe the filler neck and the cap’s seals. Grit here causes leaks.
• Testing: A workshop can pressure-test the cap and system, replace any cap that doesn’t hold spec.
• Replacement: Many techs treat caps as a 5–7 year or ~100,000 km item, or sooner if seals harden. It’s affordable insurance.
• Safety: Never open the cap hot. If topping up, use the filler/degas tank under the cap, not the overflow bottle, and bleed air per Subaru procedure. Use the correct long-life coolant compatible with Subaru blue SLLC.

Because the radiator itself has no cap on this model, owners should know all coolant filling and bleeding happens at that engine-mounted filler tank. A good cap keeps the Impreza’s cooling system sealed, pressurised, and trouble-free.

Where is the radiator cap on a 2018 Subaru Impreza?
It’s on the engine-mounted coolant filler/degas tank, near the top of the engine under the bonnet—not on the radiator. This is the system’s pressure cap and the correct place to top up and bleed when the engine is cold.

What pressure rating cap does it use?
Most 2018 Impreza 2.0L models use a cap rated about 108 kPa (1.1 bar). Check the stamp on your cap or the Subaru service information for your VIN to confirm.

How often should the radiator cap be replaced?
Inspect at every service and replace if the seal is perished, the spring is weak, or it fails a pressure test. As a preventive measure, many workshops replace the cap around 5–7 years or 100,000 km.