Coolant Dilution

[Desh_Fever]

Hello,

A buddy of mine was replacing his water bottle (from a polo 9n3 GTI) along with the coolant (VW's G12). Yesterday he came through and told me that he diluted the coolant (VW G12) with tap water.

Is tap water okay for the mixture? If not, what could possibly go wrong with his car?

[VAG Fan]

There have been minor wars on this topic. Some are fanatical about the distilled water, others have always and will always use tap water.
In my opinion as a chemist, if your tap water is not particularly hard (lots of minerals, limestone), it should be fine to use.
I've never seen any engine damage that could unequivocally be traced to the use of tap water as opposed to distilled / deionised water.

[missioner]

I have it on good authority that distilled water is actually detrimental as it can accelerate corrosion of metal components in the cooling system.

As it was explained to me, it's because of the *lack* of minerals and other chemicals that form part of natural water, tap or otherwise. The removal of the metals and minerals essentially disrupts the ionic balance and as a result the distilled water then seeks to restore this by oxidizing the surrounding materials.

I can't confirm this personally related to distilled water, but I have used tap water in filling cooling systems since 1996 without any negative effects. Neither have any of the many different manufacturers I have worked for ever advised against using tap water in the cooling systems of new vehicles, they supply concentrated coolant additives which is diluted with water when filling.

The only thing I will add to this debate is to NOT run pure concentrated antifreeze additive in a cooling system without diluting it. This becomes even more corrosive than anything else I have ever seen. It even damages the plastic parts that most modern cars use.

[Desh_Fever]

Thanks for the info -- I guess time will tell on his car...