I'd personally add as many as a few pint or so of distilled drinking water.. anymore and i would get some coolant To combine. if it goes off once more, deliver it on the seller. its very likely just evap loss of the water portion of the coolant combine. glycol https://felixpufwo.ampedpages.com/everything-about-types-of-coolant-for-bmw-335i-54771874