Subaru WRX and STI Cooling System

What the Cooling System Does

Coolant carries heat from the aluminum block and heads to the radiator. The water pump moves it. The thermostat decides when the radiator is in the loop. The fans add airflow when road speed is not enough. On a turbo car, the same circuit also cools the turbo after shutdown, which is why there is a filler tank by the turbo and a separate overflow bottle.

Water is the part that actually moves the heat. Glycol is there to push the freeze point below 32°F and the boil point above 212°F, and to carry the corrosion package the aluminum needs. A 50/50 mix is the street fill. Straight water cools harder and freezes, so it stays on a race car that is drained the same day. Straight deionized water, with no coolant in it, is corrosive. Do not run it.

An EJ STI holds about 7.7 liters (8.1 US quarts in the 2008 spec, printed as 8.2 US quarts in the 2017 spec and in the S209 manual). The overflow reservoir is extra, about 0.45 liters (0.48 US quarts). A radiator drain on an EJ20 WRX gets most of a system that size. The rest sits in the block until the thermostat opens. A neck that stops taking fluid before you have poured about 7.7 liters is waiting on that open, not telling you the system is smaller.

Which Coolant

Genuine Subaru coolant is the fill that matches the aluminum engine and the aluminum radiator. It is a phosphate, non-amine formula. Warranty work that adds coolant uses that fluid, plus Genuine Subaru Cooling System Conditioner. The bottles of concentrate and premix look almost the same. Read the label before you pour a full gallon of concentrate in as if it were already 50/50.

Two Subaru coolants show up in the real world. The older long-life coolant is the green phosphate fluid, changed about every 30,000 miles or 30 months. Subaru Super Coolant is the blue fluid. Cars that left the factory on Super Coolant follow the long interval: first replacement at 11 years or 137,500 miles, then every 6 years or 75,000 miles. Hoses and the cooling system still get inspected on the shorter schedule. An inspection is not a coolant change.

Blue can top off green. The two are compatible in that direction. Topping blue off with green drops the car back onto the 30,000-mile schedule unless the system is later exchanged on the approved machine. Putting blue into a pre-2008 car that came with green does not earn the 11-year interval. Keep the green interval. The 6-year / 75,000-mile interval applies to that older car only after a clean exchange on a Wynn’s PowerFlush III, the three-tank machine that flushes with clean water and does not reuse the old fluid.

Phosphates protect aluminum when the coolant is changed on time and mixed with soft water. In hard tap water they can fall out and leave deposits. That is why the mix water is deionized first, distilled second, and tap water last. It is not a reason to throw out Subaru coolant. Silicates are the additive to avoid in an aftermarket jug. They clean, and they also wear water-pump seals.

A silicate-free, phosphate-free extended-life jug is a different chemistry from Subaru’s phosphate fluid. Peak Global is the silicate-free, phosphate-free, non-2-EH option when the car is not staying on Subaru coolant. It still goes in after the old coolant is out, not on top of it. Chemical flush agents and stop-leak in a can are out. Subaru’s own conditioner is the sealer that was tested on these engines. Other sealers and radiator-flush chemicals can corrode aluminum, and a flush machine that has been used on copper radiators can leave copper in the system.

Cooling-system conditioner, warranty number SOA635071, is a 4.4 oz bottle. The case number is SOA345001. Pour it on every full coolant change, for every model year, and shake the bottle first so the thick part on the bottom goes in. It is for small seepage. It slowed a 5-inch crack on one radiator and still was not a repair. The radiator came apart later. A splash of water on a top-off does not get a fraction of the bottle. On one used car the vacuum valve in the cap clogged, the upper hose collapsed as the engine cooled, and the hose had already been rubbing the fan. If the upper hose collapses cold, clean or replace the cap before you blame the head gasket.

Surfactant additives (Water Wetter, Purple Ice, and the same class of product) are optional. The “minus XX degrees” on the bottle is from straight-water tests. Mixed with coolant, the drop is a few degrees. They do not replace the conditioner, and they are not required for a street car. Race coolants such as Evans or Engine Ice have their own fill rules. They are not the daily driver default.

Mix and Change Interval

Subaru Super Coolant is specified at 50 to 60 percent concentrate. That band is where freeze protection and corrosion protection both hold. Richer than 60 percent is outside the spec. A mix that is mostly water cools a little harder and leaves the corrosion package below the 50 percent floor.

Colder than those freeze points, raise the concentrate inside the 50 to 60 percent band. Do not invent a stronger mix past 60 percent. Dilute concentrate with distilled water, or with soft tap water if that is all you have. Deionized water is the first choice in the mix and a bad idea by itself.

A small top-off can be plain water, or premix if you would rather not change the ratio at all. A few ounces will not move a 7.7-liter system off the chart. If the overflow is down at the bottom and the heater has gone cold, that is not a top-off. That is a leak or an air pocket, and it gets a real fill.

Change Subaru coolant on the interval for the fluid that is actually in the car, from the table above. Another brand follows that brand’s interval only after it is the only fluid in the system. Skipping the change until the expansion tank is full of mud is how radiators clog. When the tank looks dirty, pull it, empty it, and rinse it. The conditioner itself does not turn a clean tank brown if the coolant is being changed.

Caps and Pressure

Higher pressure raises the boiling point, and it also loads every hose, gasket, and plastic tank. On the turbo cars that have two caps, the caps are not interchangeable. The filler-tank cap, the one by the turbo, is the lower rating, about 108 kPa (1.1 kg/cm², about 16 psi). It has a vacuum valve. That valve is what lets the system pull coolant back from the overflow as it cools. The radiator cap is the higher rating, about 137 kPa (1.4 kg/cm², about 20 psi), and it is pressure relief only. It is the plain cap, without wings. The winged cap is the filler-tank cap.

The 2008 spec is 108 kPa plus or minus about 15 kPa on the filler, and 137 kPa plus or minus about 15 kPa on the radiator. The 2017 spec states the same idea as a range: filler 93 to 123 kPa in service, radiator 122 to 152 kPa. A cap that will not hold the limit on a pressure tester is done. Swapping a 1.1 bar cap onto a neck that was already 1.08 bar changes nothing you can use.

Leave the radiator cap on while you fill the filler tank. The filler cap is the one that vents to the overflow in normal use. The radiator cap is the backup. Caps backward, and the system pushes coolant into the overflow, will not pull it back, and collects air in the filler tank. A missing cap is not a short drive to the dealer. The system is unpressurized, the boil point falls back toward 212°F, and a hot day or a hill will push coolant out the open neck.

A 2002 WRX often has no radiator cap. Coolant goes in the filler neck, and there is no bleed hole hiding under a cap the radiator does not have. The two-cap rule still applies the moment an STI-style radiator with its own cap is installed: higher cap on the radiator, lower cap with the vacuum valve on the filler tank.

Thermostat and Fans

The stock thermostat is a wax pellet. It starts to open at 76 to 80°C (169 to 176°F) and is fully open at 91°C (196°F), with at least 9.0 mm (0.354 in) of valve lift. The number stamped on the thermostat is the start-open temperature, not the temperature the gauge should sit at. One 2004 WRX that lived at 194 to 204°F with a thermostat stamped near 172°F was in the normal window. The valve is open, and the fans have not been asked to work yet.

Install it with the jiggle pin facing up, on a new gasket. The EJ gasket is part 21236AA010. It fits the 2004 WRX housing and the other EJ cars that use that housing, including 2002–2014 WRX and 2004–2021 STI. The 2015–2021 FA20 WRX uses a different housing, so that gasket does not transfer. Torque the cover to 12 N·m (8.9 ft-lb). A thermostat that fails shut overheats. One that sticks open takes longer to make heat, and the heater stays weak. Either way it is a new thermostat and a new gasket, not a cooler temperature rating.

A lower-temperature thermostat is for a car that is already heat-soaked on track or making enough power that the stock radiator is the limit. On a street car it opens the radiator earlier and can leave the engine below the temperature it was tuned to run. One lower-temp unit on an otherwise stock radiator sat in the 170s and still spiked after a hard pull. It did not add airflow. Fans are the airflow. On one 2006 WRX they came on at 206°F and the gauge came back to about 188°F. On the 2008 STI, low speed starts as coolant climbs through about 90 to 95°C (194 to 203°F), and high speed starts at about 96°C (205°F). On the way down the fans shut off a few degrees cooler. Road speed and A/C load change which band applies, and highway speed can leave the fans off longer because air is already going through the core. A fan that is dead, or a relay and harness that never command it, overheats in traffic no matter what thermostat is in the housing.

Keep the factory fans. A slim aftermarket fan usually gives back the cooling an aluminum radiator just added. The stock pair is sized for this core. Replacing one with a thinner unit to clear a hose is how cars run hotter after a “cooling” upgrade.

Radiator and Hoses

The stock radiator is a downflow core, about 687 mm wide, 340 mm tall, and 16 mm thick, with plastic end tanks. Those tanks are the failure. They crack, and the upper tank separates from the core. One car did it near 67,000 miles. An ’03 cracked the end tank the same way. When that happens, another plastic-tank radiator is the same part that just failed. An aluminum radiator is the replacement that removes the seam. It is not a horsepower part. It is a tank that does not split, with some extra capacity because many of them are thicker than the 16 mm core.

A thicker core, the Koyo on a 2004 WRX being the usual example, can make the upper hose kink unless the hose is trimmed so it takes a natural bend. Silicone hoses and new clamps do not fix a kink. They also do not fix a cap on the wrong neck. Replace a cracked rubber hose. The upper hose that collapses only when the engine is cold is a vacuum-side cap problem, or a hose that has gone soft, not a reason to raise system pressure.

The small hose from the turbo coolant tank to the filler neck leaks at the clamp more often than the radiator does. A new clamp there is a ten-minute job. The radiator drain plug on an aluminum radiator is easy to round. A short hose on a petcock, or a valve in place of the plastic plug, keeps the drain off the crossmember and out of the driveway.

Block drains drawn for a naturally aspirated engine do not match an EJ20 WRX. The hose at the oil cooler is not worth pulling for the last cup of coolant. Drain the radiator. If you want what is left in the block, pull the thermostat housing while the lower hose is off, then refill with the pin-up thermostat and a new gasket.

Drain, Fill, and Air

Air is the overheating problem that looks like a bad head gasket. The gauge climbs, the heater blows cold, and a block test for combustion gas in the coolant comes back clean. A shop fill uses a vacuum tool so the system is pulled down before the coolant goes in. The driveway procedure below is the EJ STI fill, and it works on the turbo WRX cars that use the same filler tank. Fill a 2015-and-later FA20 WRX at its own neck. The housing and the gasket are not the EJ parts.

1. Drain it cold. Under cover off, radiator drain plug out, filler cap off so it drains faster. Catch it. The plug goes back in before any fill.
2. Conditioner first. Shake the 4.4 oz bottle and pour it through the filler neck. Then fill the filler tank to the neck with 50 to 60 percent Subaru coolant. Leave the radiator cap on.
3. Overflow to FULL. That bottle is the reservoir, about 0.45 liters, not the place you pour the whole system.
4. Short revs, cap on. Five or six blips at 3,000 rpm or less, done inside 40 seconds. Shut it off, wait a minute, open the filler cap, and top the neck if the level dropped. Do that pair of steps twice.
5. Heater on, fans cycled. Heat at full hot, blower on low. Hold about 2,000 rpm until the radiator fan starts and then stops. Watch the gauge. If it climbs and the fan never comes on, stop and fix the fan before you keep idling.
6. Cool to 86°F and top off. At 30°C (86°F) or below, open the filler cap. Bring the neck back to full and the reservoir back to FULL.
7. Listen at the heater. Heat full hot, blower low, a few revs to 3,000 rpm. A flowing or gurgling sound in the heater core means air is still in it. Repeat the fan cycle and the cold top-off.

A spill-free funnel on an open neck, run until the thermostat opens and the level drops, is the same idea when you cannot do the timed rev sequence. On a 2002 with no radiator cap, the vent hose from the top of the radiator up to the filler tank can be raised so coolant stays in the hose while the air comes out. Half a quart lost during another repair still gets replaced. “It did not overheat on the way home” is not a bleed.

After the first heat cycle, check the level cold the next morning. Between LOW and FULL on the reservoir is the window, closer to FULL, and the hose inside the reservoir has to stay submerged or it will suck air. Coolant on the floor is a leak: a cap, a hose clamp, a plastic tank, or a water-pump weep hole. Conditioner is not the repair for a crack you can see.

FAQ

What coolant goes in a Subaru WRX or STI cooling system?

Genuine Subaru coolant at 50 to 60 percent, plus the 4.4 oz conditioner on a full change. Green long-life stays on the 30,000-mile interval. Factory blue Super Coolant uses the long interval. An aftermarket jug has to be silicate-free, and it goes in only after the old coolant is flushed out.

Can I mix the blue and green Subaru coolants?

Blue can top off green. Green on top of blue puts the car back on the 30,000-mile interval. A pre-2008 car filled with blue keeps that shorter interval unless the exchange was done on a Wynn’s PowerFlush III.

Is 200°F too hot?

No. Full-open on the stock thermostat is 196°F. A gauge in the high 190s with heat at the vents is a system that is working.

Which cap goes on the radiator?

The plain 137 kPa cap, pressure only. The winged 108 kPa cap, the one with the vacuum valve, goes on the filler tank by the turbo. Leave the radiator cap on while you fill.

Do I add conditioner when I top off?

No. The full bottle goes in when the coolant is changed. A few ounces of water or premix is enough for a small top-off.

Will a cooler thermostat fix high coolant temps?

Not by itself. It opens the radiator sooner. Airflow still comes from the radiator and the factory fans. A street car that is not heat-soaked does not need a lower opening temperature.

Can I run straight water?

Only in a race car that will be drained. It freezes at 32°F, and deionized water with no coolant in it attacks the system. Street cars stay at 50 to 60 percent Subaru coolant.