Many turbo Subaru vehicles have 2 caps on the cooling system: one on the coolant expansion tank and one on the radiator. These are very important in the operation of the cooling system, and each cap has a very specific purpose. If these caps are not used in the proper place, or the wrong cap is used, it can cause problems with your cooling system. These problems can become increasingly dangerous in higher horsepower vehicles because they create more heat.
On a Subaru, The cap on the coolant expansion tank acts similarly to the radiator cap of a single-cap cooling system. When the car heats up or overheats the coolant will need room to expand. The cap has a built-in pressure release valve, set at 1.1 BAR (108 kPa) of pressure, that leads the coolant to flow into the overflow tank. When the Vehicle cools down the coolant will contract creating a vacuum in the system. An additional valve in the cap will respond to the vacuum and allow coolant from the overflow to be pulled back into the system. This is known as a 2-way cap and is common on many vehicles cooling systems. The expansion tank is also the high point of the system and should be the part of the system where you bleed air from.
Unlike a traditional radiator cap the Subaru radiator cap is only a 1-way. This means it will allow coolant to escape when the system is under pressure but does not have the secondary valve to recover coolant as the car cools off. The pressure rating on this cap is also set higher than the cap on the expansion tank, at 1.3 BAR (137 kPa). This cap is only used as a safety measure in the event of overheating, to relieve pressure from the system. This cap should not be removed when bleeding/burping your cooling system.
If these caps happen to be switched or replaced with the wrong caps, your vehicle may overheat. If your vehicle does experience overheating then the factory built-in recovery feature will not function correctly and the problem could persist. To ensure your cooling system works properly you need to have a 1.1 BAR/108kPa 2-way cap on the expansion tank near the turbo. The radiator needs to have a 1.3 BAR/137 kPa 1-way cap. If you have Subaru OEM caps the easy way to tell is; the winged cap goes on the tank, the round cap goes on the radiator. Most aftermarket caps are 2-way caps. If you plan to get an aftermarket cap, we suggest you keep the OEM round cap and use a 1.1 BAR cap on the expansion tank.