Parts for your 2004 Subaru Impreza-Radiator cap

2004 Subaru Impreza radiator cap — what it does and how to look after it

Technical references confirm a radiator cap is absolutely relevant on the 2004 Subaru Impreza. The Subaru Factory Service Manual for MY2004 Impreza (Cooling System section) specifies inspection and test pressures for the cap, and Subaru’s electronic parts catalogue (FAST) lists a genuine pressure cap for all 2004 Impreza variants. On naturally aspirated models the cap sits on the radiator neck, on WRX/STI it’s on the pressurised filler/header tank near the intake manifold. Genuine Subaru caps are typically rated around 1.1 bar (approx. 108 kPa) — always match the rating shown on the vehicle or by VIN in the parts catalogue.

The radiator cap is a clever little pressure valve that lets the cooling system run at a higher pressure so the coolant’s boiling point goes up. That keeps things stable on hot days, steep climbs, or long motorway runs. It also has a vacuum valve to pull coolant back from the overflow bottle as the engine cools, preventing hoses from collapsing and keeping the system full.

As part of routine servicing on a 2004 Impreza, it’s smart to inspect or replace the radiator cap whenever coolant is changed, or at least every 2–4 years. Look for hardened or cracked seals, corrosion on the seat, weak spring tension, or staining that hints at leaks. A cooling-system cap tester can confirm whether it holds the specified pressure and releases correctly.

• Common symptoms of a dodgy cap: random overheating at speed, coolant loss into the overflow bottle, collapsed hoses after cool-down, or slow warm-up with poor heater performance.
• Replacement tips: only swap caps when the engine is stone cold, clean the filler neck, fit the correct pressure rating, and for WRX/STI fill/bleed from the header tank, not the overflow bottle.

While some aftermarket “high-pressure” caps exist, going above the specified rating can stress hoses, radiator tanks, and the heater core. Sticking with a quality, OEM-spec cap keeps the EJ engine happy and the cooling system reliable for Kiwi and Aussie conditions.

Popular questions

What pressure radiator cap does a 2004 Impreza use?
Most 2004 Impreza models use a cap around 1.1 bar (about 108 kPa). WRX and STI commonly share this rating. Always confirm by checking the marking on your existing cap or by VIN in the parts catalogue to avoid mismatching components.

Where is the radiator cap on a 2004 Impreza?
On non-turbo models it’s on the radiator’s filler neck. On WRX/STI, it’s on the pressurised filler/header tank mounted high near the intake manifold, the radiator itself often doesn’t have a cap.

How do you test a radiator cap?
Use a cap pressure tester with the correct adaptor. Pump it up to the spec and confirm it holds and then releases near the rated pressure. Also check the rubber seals and the seat for wear or corrosion. If in doubt, replace — caps are inexpensive insurance.