Originally Posted by mashimarho
Would distilled water be safe without 50/50 mix or anything?
yes. The expansion tank your adding to holds a small amount of coolant relative to the total amount in the system. What you're trying to do is top it off when it's totally cold to the max fill line, then monitor it. If you keep losing coolant there is either a leak or a head gasket situation. Then you can figure out if you need to take it in. in which case you will have to address the tune.