I might add as many as about a pint or so of distilled water.. anymore and I'd personally pick up some coolant to mix. if it goes off all over again, convey it towards the supplier. its probable just evap loss of the h2o part of the coolant blend. glycol https://autoworks16048.educationalimpactblog.com/49525114/the-basic-principles-of-types-of-coolant-for-bmw-335i