[DokiBMW]

Today I went for a drive, about 5 kilometers. When I got home, I heard a hissing sound, and when I opened the hood, coolant was coming out of the expansion tank. Almost all the coolant is gone now. I have to admit, I drove fast (yes, the engine was warm because I had already driven 100 kilometers earlier in the afternoon). It was very hot today (around 30 degrees Celsius). What could be the problem? Help! Thanks in advance.

[No one]

Driving fast is good for cooling (with the oncoming air). High revving - not so much.

If the expansion tank is not damaged make sure it is closed properly, perhaps: "Fit the cap and turn until it is clearly heard to click into place." (the manual, page 251).