I might add as many as a couple of pint or so of distilled water.. anymore and i would pick up some coolant To combine. if it goes off all over again, carry it towards the dealer. its possible just evap loss of the water percentage of the coolant combine. https://automotive-collision-repa59146.isblog.net/types-of-coolant-for-bmw-335i-an-overview-44408731