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The back of the Audi G13 coolant concentrate bottle I have specifies "use with distilled water ONLY", is it OK to use deionized water instead, or any other kind of water?

I’ve read a great deal of opinions on this topic but am looking for the facts, opinions on this matter vary wildly from "it doesn't matter" through to "you should only ever use [distilled or deionized] water" (and opinion varies on which is more pure), some even say that tap water is preferable!

Distilled water is quite scarce in the UK at least, instead everywhere will sell you deionized water and will treat both as one and the same, when in fact according to Wikipedia they are different.

Things are made more confusing when you look online to purchase distilled water and discover that manufacturers boast that it has also been through reverse osmosis and deionization!

  • I'm not sure where you are located, but in my area distilled water is easily available in the local food mart. Keep in mind that the Audi G13 advice is specifically for that coolant. Each different brand/formulation may have it's own requirements. I've had vehicles in the past that caution against using distilled water. – Tim Nevins Oct 31 '18 at 20:08
  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Oct 31 '18 at 21:19
  • @TimNevins I'm in the UK - it seems that you guys in the US have distilled water everywhere but here we just don't seem to use it as much. We can buy water for steam irons but that's deionised too generally, not distilled. – Mike Whitby Nov 1 '18 at 13:30
  • @MikeWhitby I'm sure it's exactly the same thing, whether you buy it from Tesco or Walmart, and whether it's called deionized or distilled. These places aren't laboratory supply stores. – Al_ Nov 1 '18 at 15:01
  • Not being clear on the differences between distilled and deionized water I looked it up. They are similar but not identical. One thing stuck out; Because it is corrosive, deionized water is not used in situations involving long term contact with metals. – Tim Nevins Nov 5 '18 at 15:30
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Usually distilled water is purer (has more of its minerals and organisms removed) than deionized water. So better choice is distilled, but your will be fine with deionized as well. However if the coolant user manual says to use distilled water I advice you to use distilled.

  • The funny thing is that some sources online seem to state that distilled is more pure, and others state that DI is more pure, it's very confusing! – Mike Whitby Nov 1 '18 at 13:32
  • @MikeWhitby It's because purity is on demand. Making water close to absolute purity is an expensive process. You're not doing a lab experiment here. You're just filling an automotive cooling system with coolant, so the distilled water can be not so thoroughly distilled without any problem. – Al_ Nov 1 '18 at 14:32
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    Agree with @Al_ here... once it goes down the filler neck, it's going to pick up all kinds of minerals anyway :) – Zaid Nov 1 '18 at 14:42
  • @Zaid OAT/HOAT coolant apparently cleans rust away from the heat exchanging surfaces in the system and make it circulate about. That's the "brown sludge". If you see any, look for leaks, air bubbles or faulty cap. And always keep the system full so no air pockets develop. – Al_ Nov 1 '18 at 15:17
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It's always better to follow OEM indications to the letter, whenever possible. So if it says distilled water, try to get distilled water and nothing else.

That said, in the end i guess it doesn't matter, as long as you never use distilled or deionized water alone (not mixed with coolant) in the cooling system. That one can effectively react with the aluminum in the system and corrode/cavitate it over time. The coolant is formulated to prevent these reactions and protect the system when mixed with water (some coolants state that tap water can be used also), and that's also why the coolant/water mix has to be replaced from time to time (5 years generally for the OAT formulated coolants, 2 years for the green silicate-based ones) even if stated as long-life, since the formulation eventually wears out.

Wanna be completely sure and not care about the distilled water/deionized water/tap water issue at all? Buy premixed coolant.

  • "The coolant is formulated to prevent these reactions and protect the system when mixed with water" This is what I've read and so I do wonder if I'm being over the top, try as I might it seems quite hard to get water that has been solely distilled, when looking online places that purport to sell distilled water boast about the fact it's also gone through RO and deionisation treatments, which I don't want! – Mike Whitby Nov 1 '18 at 13:32
  • We're not talking of a laboratory. So, as long as not too many Calcium and Carbonate ions are there, you're fine. You don't need absolute purity, especially when we're talking of a coolant/water solution and the coolant comes with specific additives meant to preserve the coolant system integrity. Just never use distilled or deionized alone, because it eats through aluminum over time. Have a look here: thoughtco.com/distilled-versus-deionized-water-609435 . It clearly states that deionized water is used for: "cooling applications where it's important to avoid depositing minerals". – Al_ Nov 1 '18 at 14:22
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Ok so the other answers are all valid but I'll answer my own question as I got in touch with 3 Audi dealers, they each gave three different answers (fantastic!), they were:

  1. De-ionized and distilled are different names for the same thing, use either
  2. Just use tap water
  3. Internally we use Audi coolant mixing water, but we used to use tap water

So, it seems like tap water is readily on the options list, or deionized water. In short - they don't seem to give much of a monkeys, however Audi is clamping down on using the proper water (so dealership #3 told me) so they now use (and sell) mixing water for this purpose.

So I think the correct, by-the-book way is to use the Audi water. I'll buy some and update my answer as to what it is (if I can find out).

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