Originally Posted by cassiri
So the coolant level is very slightly low. The cap says "water" in many languages. Do you add just water to bring it up to level? Is the white crusty residue around the cap normal?
As I mentioned before, is coolant loss normal?
It's OK to add distilled water to top off the coolant. Tap water contains stuff that can cause corrosion in the cooling system.
Gradual coolant loss seems to plague Boxsters & Caymans with no apparent single cause. There are numerous threads on this topic on the Planet 9 forum, so do a search there. The crusty white residue around your cap could mean the cap can't hold pressure and needs replacement.