Porsche Enthusiasts Club Forum

I have read that is best to use distilled water mixed with antifreeze. Is this correct?
My independent specialist has never heard of it.

So coolant you buy ready mixed will have water added to it, coolant that needs diluting typically 25/75 or 50/50 will need water. In theory I don't suppose it makes a difference. And with old cast iron engines & copper/brass radiators it probably didn't. But modern cooling systems are so finely tuned all that hardwater cannot do it any good.
I have a dehumidifier at home so always use that in cooling systems & washer bottles.

Part of the day job involves coolants and these are not only factory premixed but filtered to very fine limits. Typically 25 microns, which is way in excess of domestic water.

So in short although you'll probably get away with it, distilled must be better than tap water especially if in a super hardwater area such as mine.