On Thu, 29 Apr 2004, Daniel L. Katz wrote:

> maybe we can get a chemist to comment; but, it seems logical that
> distilled water would actually be more corrosive than average tap water.
> there may be some advantage in terms of reducing mineral deposits, but
> distilled water should really be more corrosive because it is farther from
> chemical equilibrium with respect to cooling system materials.

In fact, tap water's pH is usually adjusted at the treatment plant to make
sure it isn't corrosive to the water mains.  (This isn't true if you have
your own well, of course.)

The concern with tap water is that the minerals in it could react with
chemicals in the coolant and cause them to settle out, forming deposits
that can clog the system.  This is supposedly why VW specified
phosphate-free coolant.

David Brodbeck, N8SRE
'86 Volvo 240DL wagon
'82 VW Vanagon Westfalia Diesel