By the time your deionized water is heated in your engine block for the first time, it's been completely reionized by whatever metal ions it can get (and it will get them). My gut feeling is after the water has its ions to reach an electrochemical equilibrium the water will be as good as regular distilled water.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.