Parts for your 1998 Subaru Forester-Radiator cap

1998 Subaru Forester radiator cap — what it does and when to replace it

Technical verdict: the 1998 Subaru Forester does use a radiator cap. The Subaru Factory Service Manual for the 1998 model year Forester (Cooling System section) specifies a radiator cap opening pressure of approximately 108 kPa (1.1 bar). Subaru’s genuine parts catalogue for SF-series Forester also lists a cap for the radiator, and mainstream aftermarket application guides (e.g., Gates and Stant) include a dedicated cap for the 1998 Forester. So the radiator cap is absolutely relevant to this vehicle.

On a 1998 Forester, the radiator cap does much more than keep coolant from splashing out. It’s a pressure regulator that raises the boiling point of the coolant, helping the EJ engine hold its temperature on hot days and steep climbs. It also acts as a two-way valve: as the coolant heats and expands, excess pressure is vented to the overflow bottle, as the system cools, the cap’s vacuum valve draws coolant back in so there’s no air sneaking into the system under the bonnet.

When the cap gets tired—spring weak, seals hardened, or the valve sticky—the system can’t hold pressure. That can show up as creeping temps, coolant loss into the overflow, collapsed upper hose after cool-down, or discoloured coolant. Because a cap is inexpensive and critical, it’s smart maintenance on an older Forester.

Service advice for this model:

• Inspection cadence: check the cap at every service (about every 10,000 km) for cracked rubber, corroded seat, and weak spring. Replace every 3–5 years, or sooner if there are symptoms.
• Spec: most 1998 Forester variants call for a 1.1 bar (approx. 108 kPa) cap. Always confirm on the cap label, radiator support decal, or FSM.
• Testing: a cooling-system pressure tester with a cap adaptor will quickly show if it holds rated pressure and releases smoothly.
• Replacement tips: only remove when stone cold. Press down and turn slowly to the first stop to bleed any residual pressure, then remove. Wipe the filler neck clean, check for nicks, and fit the new cap squarely. After any cooling work, top up to the correct level and burp the system so there’s no trapped air. Use the proper coolant mix (typically 50/50) and note the level in the overflow bottle sits between LOW and FULL when cold.

Keeping the radiator cap up to scratch helps the Forester’s cooling system stay sealed, pressurised, and reliable—exactly what’s needed for daily runs and long hauls across Aussie and Kiwi roads.

Popular questions about 1998 Subaru Forester radiator caps

What pressure radiator cap should a 1998 Forester use?
Most 1998 Forester models specify a 1.1 bar (about 108 kPa) cap, as noted in Subaru’s service data for the SF series. It’s best to match the factory rating printed on the original cap or under-bonnet label, especially if the vehicle has had cooling-system parts changed over the years.

If in doubt, ask for a cap by vehicle application (1998 Subaru Forester, engine code EJ20/EJ25) and confirm the rating on the new cap before fitting.

How can someone tell if the radiator cap is failing on a ’98 Forester?
Common clues include coolant pushing into the overflow and not returning when cold, a collapsed top hose after shut-down, unexplained coolant loss, or rising temps under load. A visual check may reveal cracked seals or a corroded seat.

A quick pressure-cap test with the right adaptor will confirm whether it holds its rated pressure and releases correctly.

Is it safe to upgrade to a higher-pressure cap?
Going above the factory rating isn’t recommended. A higher-pressure cap can stress hoses, the radiator core, and plastic tanks, risking leaks. Subaru’s specified pressure balances boil-over protection with component longevity.

Stick with the factory-rated cap and focus on clean coolant, good hoses, and a healthy thermostat and water pump for the best reliability.