You meant deionized water, not ionized.
You want low ion concentration in this water - ideally only the water's own H3O+ and OH- ions. Some contaminating metals, carbonate, organic ions etc. will negatively affect the engine's aluminum alloy and/or precipitate with coolant's buffer or preservative.
This is why the engine grade water is either purified by distillation or depleted by passing through ion exchangers.
Last edited by Megalocnus; 03-28-2010 at 04:58 AM.