Parts for your 2021 Subaru Impreza-Radiator cap

2021 Subaru Impreza radiator cap — what it does, where it lives, and how to look after it

Technical sources for the 2021 Subaru Impreza (FB20 engine) show the system uses a pressurised cap, but it’s fitted to the engine’s coolant filler/header tank rather than on the radiator neck itself. The Subaru Owner’s Manual for this model directs coolant filling at the “coolant filler tank” and warns against opening the cap when hot, and the Subaru Service Manual (Engine Cooling section) diagrams a radiator with no cap and a pressure cap on the filler tank. So, while the radiator core doesn’t wear a cap, the vehicle absolutely uses a pressure cap that performs the classic “radiator cap” job.

On this Impreza, that cap is the gatekeeper for system pressure and coolant flow. It seals the system so pressure can rise as the engine warms, lifting the boiling point of the coolant and keeping temperatures stable on long Kiwi and Aussie drives. It also contains two valves: one lets expanding coolant head to the overflow bottle, and another draws coolant back in as things cool down, keeping the system full and air-free.

• Raises boiling point by holding pressure, helping prevent overheating.
• Controls expansion and contraction via overflow and return.
• Maintains a proper seal to stop air ingress and coolant loss.

As part of routine servicing, that cap deserves a quick once-over. Rubber seals should be supple, not cracked or flattened. The spring should feel firm, and the underside should be clean—no crusty deposits or corrosion. If there’s unexplained coolant loss, hard top hoses when stone-cold, hoses collapsing after shutdown, or frequent overheating, the cap is a prime suspect.

• Replace preventatively every few years or around 80–100,000 km, or at the first sign of seepage or perishing.
• Only use a quality cap with the exact pressure rating specified for the car (check the original cap and the owner’s manual).
• Never open the cap when hot, wait till fully cool, then open slowly to release any residual pressure.

1. With the engine cold, remove the cap from the coolant filler/header tank under the bonnet.
2. Inspect the rubber gasket and the seating surface, clean any light residue.
3. If replacing, match the cap’s pressure rating, fit the new cap squarely and tighten till it positively clicks/seats.
4. Top up using the correct Subaru long-life coolant (blue) pre-mix, run the heater, idle the engine, and top the overflow bottle to the mark after any bleeding.

Sticking with genuine or OE-equivalent quality is smart, and mixing coolants or dropping the pressure rating can cause dramas like boiling, aeration, and premature component wear.

Where is the radiator cap on a 2021 Subaru Impreza?

It’s on the coolant filler/header tank near the top of the engine bay, not on the radiator itself. Subaru routes filling and pressure control through that tank, so that’s the cap to use for checks, topping up, and pressure-related diagnostics.

What are the signs the cap needs replacing?

Look for perished or flattened rubber seals, white or rusty deposits on the cap, coolant stains around the filler neck, hoses collapsing as the engine cools, or recurring overheating and coolant loss with no obvious leaks. Any of these can point to a weak spring or a valve that’s not sealing properly.

What pressure rating cap should be used?

Use the exact rating printed on the original cap and specified in the owner’s manual or service data for the FB20 engine. Many naturally aspirated Subaru caps are around 1.1 bar, but the correct rating for the vehicle should always be matched to maintain proper boiling margin and system operation.