So the overflow tank receives the excess liquid coolant from expansion – it’s an expansion tank.
The overflow tank should be at the full mark when cold and above the full mark when hot. When the engine cools back down, the overflow tank level is supposed to return to the full mark. What goes out comes back in, except for a small amount of evaporation which would not be visible in a week’s time. Over several months you might notice that the overflow tank it’s quite full anymore when cold due to evaporation.
The main thing is to never let the overflow tank go below the “low” mark because below the low mark, the radiator may suck in air. Any level between full and low (when cold) is safe.
Here’s my overflow tank in a 2020 Outback but the principle for overflow tanks has been the same for decades even though the shape of the tank may change with various models. The overflow tank shown here is when the engine is stone cold. This is how it came from the dealership when I bought it. When the engine is fully warmed up, the coolant level reaches the yellow mark on the tank. I didn’t make the yellow paint mark – it came that way when I bought the car. It seems that most new Subarus have the yellow mark but for some reason some people don’t have it. Maybe it got washed away?
If the only cooling system problem is that the overflow tank doesn’t suck fluid back in, then there are several possibilities.
- The bottom of the tube in the overflow tank is touching the bottom of the tank, preventing it from sucking fluid back in. Subaru recommends cutting the tube back enough so that it does not touch the bottom, and leaving a 45 degree angle at the tip to prevent the possibility of it sucking itself to the bottom of the tank. If the hose is replaced it might be too long.
- The hose leading from the radiator to the overflow tank is old and has loosened. Old rubber hose loses elasticity and grip, so while it’s continues to maintain contact with the radiator, and seems fine, it’s only tight enough to contain liquid to push into the overflow tank, but not tight enough to prevent air from being sucked in when the engine cools. Preventing air from getting sucked in takes a tighter fit than preventing water from leaking. The solution would be to replace the hose with a new one.
- The hose leading from the radiator to the overflow tank is old and has micro-cracks small enough not to leak liquid but large enough to suck in air. The solution would be to replace the hose with a new one.
- The radiator cap is defective and while it allows liquid out, it’s not letting liquid get sucked back in. Replace the radiator cap with a new genuine Subaru radiator cap for your vehicle.
- The radiator itself, where it attaches to the overflow hose is damaged or cracked.
If you have gasses entering the cooling system for a different reason then that’s a whole nother thing.
