[JFP in PA]

Originally Posted by Heiko

Ok point taken; I also use Distilled water, or at least when I have it available. Our water here is very good and I often forget that other area's aren't as lucky and have a lot more crap in their water. I don't think having good quality tap water in the cooling system will hurt anything, especially if you change out the coolant on a regular basis.
Many folks refuse to change their coolant every 2-3 years like it should be done, and if it was done on a regular bases it probably wouldn't have as much of an impact having a few liters of tap water in the system; but leaving the coolant in the system for 4-5-6 or more years is probably more risky as the minerals will begin to crystalize over time and the PH level drops making the antifreeze more acidic.
In my opinion not worth flushing out the whole system because of tap water added to the system (especially if it was good clean water) - just change it in a year or two again.... but that's just me.
H

We have run the factory coolant, mixed with distilled water, for as long as seven years before any change in pH, clarity, or freeze point; and the coolant passage internals on those engine's were spotless as well. Using distilled water can not only lengthen the expected life of the coolant, but dramatically reduce the system build up as well.