I would insert as much as about a pint or so of distilled water.. any more and I'd get some coolant to mix. if it goes off once again, deliver it for the supplier. its very likely just evap lack of the drinking water portion of the coolant mix. glycol https://what-are-spoilers-on-a-ca38271.blogmazing.com/26970549/5-simple-techniques-for-types-of-coolant-for-bmw-335i