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Never seen, or at least never noticed, any de-ionized water in the store here. I have always used the distilled though and never had any problems with a fresh kept cooling mix. I know better than to use tap water here in San Antonio. We've got just about the hardest water on the planet! You can fill a glass with ice, made from local tap water, and let it melt. The bottom of the glass will be full of crud that looks like dandruff ! I think freezing makes it dump out a lot of the lime and calcium in the water here so you can see what is really in there!
Never seen, or at least never noticed, any de-ionized water in the store here.
Again, sorry I should have never used the term "De-ionized". it comes from many years of having a De-I system in facilities it work within.
Distilled water is available at the grocery store.
as far as radiator are concerned De-I or Distilled will serve the same purpose and there should be no distinction from either one.
So if you have De ionized water in an Electronics manufacturing facility or something similar, then use that if it is free. otherwise buy the Distilled water at the grocery store or make a still in your back yard and process your own
If I had a still, I don't think I would be making water. I'd be making an elixir for the body. Actually something more along the lines of the "Recipe".
I'm glad this was posted, I need to change mine out soon. It's been about 2 years but what the heck, I have been changing all of the fluids so might as well change the radiator fluid as well.