2021 Subaru Forester Coolant Capacity & Type: A Complete Guide

The 2021 Subaru Forester holds approximately 8.2 quarts (7.8 liters) of engine coolant. As an ASE-certified master technician, I regularly see these vehicles in the bay, and properly maintaining the cooling system is critical to the longevity of your flat-four "boxer" engine. Whether you are doing a routine top-off or a complete system flush, using the right fluid and bleeding procedures is essential to prevent catastrophic overheating.

Quick answer

Here are the essential cooling system specifications for the 2021 Subaru Forester:

• Total Cooling System Capacity: 8.2 US Quarts (7.8 Liters)
• OEM Coolant Spec: Subaru Super Coolant. This is a blue-colored, Phosphated Organic Acid Technology (P-OAT) formula specifically designed for aluminum boxer engines.
• Mix Ratio: 50/50 pre-mixed (do not add water to pre-diluted Super Coolant).

Capacity by engine option

For the 2021 model year, Subaru streamlined the Forester's engine lineup, offering only one powertrain.

Note: Total capacity includes the heater core, engine block, radiator, and reservoir tank. A standard drain-and-fill will usually require slightly less than the total capacity.

Flush interval

Subaru's blue Super Coolant is a genuine long-life fluid, but it doesn't last forever.

• Normal Duty: Subaru recommends the first factory coolant replacement at 137,500 miles or 11 years, whichever comes first. Subsequent replacements should happen every 75,000 miles or 6 years.
• Severe Duty: If you frequently tow, drive in heavy stop-and-go traffic, or live in extreme climates, we recommend inspecting the coolant at 100,000 miles. If it appears cloudy or has lost its bright blue color, flush it early to protect your water pump and gaskets.

How to refill & bleed

Because of the horizontal layout of the boxer engine, air pockets can easily get trapped in the cooling system. Trapped air will cause the engine to overheat and the heater to blow cold air.

1. Drain the old fluid: Open the petcock at the bottom of the radiator.
2. Set up a spill-free funnel: Attach a specialized burping funnel (spill-free funnel) directly to the radiator filler neck. Do not fill from the plastic overflow reservoir.
3. Fill the system: Pour 50/50 Subaru Super Coolant into the funnel until the radiator is full and there is a steady pool of fluid in the bottom of the funnel.
4. Run the engine: Start the vehicle and turn the cabin heater to the maximum temperature with the lowest fan speed. This opens the heater core circuit.
5. Bleed the air: Let the engine idle up to operating temperature. You will see bubbles burping up into the funnel. Gently squeeze the upper and lower radiator hoses to help force trapped air out.
6. Top off: Once the thermostat opens (the lower radiator hose gets hot) and no more bubbles appear, turn off the engine. Remove the funnel, install the radiator cap, and fill the plastic overflow tank to the "FULL" line.

Tech Tip: A vacuum-fill tool is highly recommended for modern Subarus to completely eliminate air pockets from the start.

Common cooling system issues

The 2021 Forester belongs to the fifth generation (SK platform). While the head gasket issues of early 2000s Subarus are long gone, this generation has a very specific and highly common cooling system issue:

• Thermo Control Valve (TCV) Failure: Instead of a traditional wax-pellet thermostat, the 2019+ Forester uses an electronic Thermo Control Valve to regulate coolant flow. The sensor inside this valve is highly prone to internal failure. If your dashboard lights up like a Christmas tree and throws codes like P26A3, P26A5, P26A6, or P0115, your TCV has likely failed.
• Water Pump Weepage: While less common than the TCV issue, occasionally the factory water pump gasket will develop a slow, crusty blue weep around 80,000 to 100,000 miles.
• Radiator Seam Leaks: The crimped plastic tanks on the OEM radiator can eventually split or leak with age and thermal cycling.

If you suspect a failing TCV or need a professional coolant flush, don't risk overheating your engine. Find a trusted shop on Repairius.