That is were the coolant goes when it expands. Sounds like coolant isn't circulating or it is being forced out of the system.
i have a 95 ford mustang and the overflow tank keeps going empty is that normal?
The car normally pushes it out the over flow.
put coolant into my over flow car was running hot and the coolant erupted like a volcano on the side of the over flow
To drain the over flow from a tank or fixture
in the over flow tank
Why dose my 232i overheating
Well if you over fill it at the gas stand of course it will over flow, if you fill it to tyhe brim and replace your cap it will not over flow - unless your filler cap has a faulty seal, if it has a faulty seal the gas is likely to spill out while driving.
an arm
Check the thermostat.
There are a few things that you could check if your BMW is overheating. You should start with your thermostat and expansion tank.
Fill the over flow tank to where it should be and just run the engine as usual. The system bleeds itself, Just check the over flow tank to make sure there is enough coolant in it.
Most of the time, when the engine is cool (not warm or hot), it should be filled to the line indicated on the over-flow tank. The anti-freeze over-flow tank is located on the passenger side with a large round top.