Parts for your 2008 Subaru Impreza-Radiator cap

2008 Subaru Impreza radiator cap — purpose, care, and fitment

Per Subaru technical documentation — specifically the 2008MY Impreza/WRX Factory Service Manual (Cooling section) and the Subaru Electronic Parts Catalogue for GE/GH chassis — the 2008 Subaru Impreza uses a pressurised radiator cap. On most non‑turbo models the cap sits on the radiator neck under the bonnet, on WRX/STI variants the pressurised cap is commonly located on the engine‑mounted coolant filler/header tank, with the overflow bottle remaining non‑pressurised. Either way, a proper radiator cap is part of the system and isn’t optional.

The radiator cap’s job is deceptively simple but absolutely crucial. It seals the cooling system and holds a set pressure (often stamped on the cap), which raises the coolant’s boiling point so the engine can run efficiently without boiling over — even on scorching Aussie or Kiwi summer days. Inside the cap are pressure and vacuum valves: the pressure valve vents excess to the overflow when things get hot and expand, the vacuum valve draws coolant back as the system cools, preventing hose collapse and air pockets.

For the 2008 Impreza, a cap rated around 1.1 bar (108 kPa) is typical on non‑turbo engines, turbo models may specify a different rating. The exact spec should match what’s printed on the existing cap or shown in the service data for the VIN. Using the wrong pressure can cause chronic coolant loss, overheating, or leaks.

Servicing advice is straightforward and doesn’t need fancy tools:

• Inspect the cap at every service or at least annually: check the rubber seal for cracks or flattening, the spring action, and the neck it mates to for nicks or corrosion.
• Pressure‑test the cap if there are symptoms like unexplained coolant loss, hard upper hose when cold, slow warm‑up, or a collapsed lower hose after shutdown.
• Replace the cap every 5 years/100,000 km, or immediately if it fails testing or shows damage. Stick with OEM or an equivalent cap with the correct rating.

When fitting a new cap, make sure the engine is stone cold. Wipe the neck clean, seat the cap squarely, and lock it fully. If your Impreza has a separate filler/header tank (common on WRX/STI), fill and bleed there as per the manual, heater on hot, topping up with the correct Subaru‑approved coolant mix. A few minutes here helps avoid airlocks and keeps temps rock‑steady on city runs and weekend missions alike.

What pressure rating is the radiator cap on a 2008 Subaru Impreza?

Most non‑turbo 2008 Imprezas use around 1.1 bar (108 kPa). Turbo variants may specify a different rating. Always check the stamping on your existing cap or the service information for your exact engine and market.

Where is the radiator cap on a 2008 Impreza?

On non‑turbo models it’s typically on the radiator neck. On WRX/STI, the pressurised cap is usually on the engine‑mounted coolant filler/header tank, while the overflow bottle is not pressurised.

How often should the cap be replaced?

Inspect yearly and replace about every 5 years/100,000 km, or sooner if it fails a pressure test or shows worn seals, weak spring tension, or corrosion.