I'm trying to change coolant on my car. I saw a video of my car where they drained the existing coolant, added and ran distilled water, drained it and then finally added coolant.

Why are they doing the distilled water step?

My owners manual doesn't say to do the distilled water step, is it necessary or a good idea, can I skip it?

I think it the answer might have something to do with impurities. If so I was low on coolant and I mixed 1 liter of coolant with 2 liters of tap water and drove for 1.5 hours. I understand this can cause minerals to get into the cooling system. Again, is a drain and replace ok or should I really be doing the distilled water step?

  • I'm rather surprised an owners manual says anything at all about the procedure. In any case the normal routine maintenance fluid change procedure assumes there is not some gross contaminant that needs to be flushed out. If there is known junk or its been an excessively long time since last change, then flush it.
    – agentp
    Commented May 10, 2018 at 14:31
  • This video demonstrates how it actually can take a couple of flushes to get a clean cooling system: youtube.com/watch?v=s--5ft5YiHg Once you get it nice and flushed clean you can switch to HOAT coolant and never have to change your coolant again for the life of the car: youtube.com/watch?v=qYX4kRaOvTM
    – Ruminator
    Commented May 10, 2018 at 18:43

4 Answers 4


When you drain the cooling system on a car it doesn't completely empty it, you'll only get is 40-60% out depending on the car. This means part of the water you put into the radiator for the flush will stay in the cooling system, if you use tap water and flush it then 40%-60% of your radiator will be filled with it after the last drain, and as you want a 50/50 mix that basically means all of the water in the system will be tap water, with all the minerals and impurities which are bad for your car. Using distilled means that after the last drain you'll have distilled only in your radiator.

  • Certain coolants come premixed with 50% water and 50% antifreeze, based on what you have said then when adding such a product,the water in there will take the water ratio up so it will become vast majority water and minority coolant. So wouldnt ot be bad to add a premixed solution and shouldn't one just be adding pure antifreeze instead? Commented May 9, 2018 at 10:24
  • Yes, if you've used distilled to flush then you'd want to use pure antifreeze. It's better to have a higher ratio of anti-freeze to water than lower, if you end up with 60% antifreeze you're fine. If you want to top up a 50/50 mix then premixed is fine.
    – GdD
    Commented May 9, 2018 at 10:30

Distilled water pH is acidic, tap water is generally neutral and some water is more alkaline. The more minerals the more alkaline the less mineral the more acidic. Distilled or demineralised water does not contain any vinegar.

  • How is this answering the question?
    – Zaid
    Commented Oct 29, 2018 at 7:03
  • And stating distilled water is "acidic" is really a misnomer. It is so slightly acidic as to be negligible. Its pH is between 6.9-7.0, with 7.0 being neutral. While this doesn't really answer the question, it doesn't provide useful information, either. Commented Oct 29, 2018 at 13:43
  • Possibly meant as a comment to this answer: mechanics.stackexchange.com/a/54180
    – Mathieu K.
    Commented Sep 1, 2022 at 16:02

Using distilled water is a garage mechanic tale in trying to use "pure" water that won't create deposits (if you regularly flush your system deposits are not an issue). However, distilled water is actually bad -

when water is distilled, or “stripped,” of its minerals and impurities, the resulting solution is composed of chemically imbalanced “ions.” This leaves distilled water “ionically hungry,” so it will actually strip electrons from the metals in a cooling system as it attempts to chemically re-balance itself. As it chemically removes electrons from the metals of cooling system components, distilled water eventually does extreme damage that could lead to cooling system failure.

Chemically speaking, this explanation is spot on, but it isn't from me, it's from the engineers at Hy-per Lube. https://www.hyperlube.com/blog/blog/why-you-should-never-use-distilled-water-in-your-cooling-system/



Distilled water has vinegar.. Vinegar breaks down crystals rust scale and so on- yes... Its very important to flush a radiator every once in awhile- kits are sold at auto parts stores as well as better flushing chemicals and agents for this purpose including one that you hook to a water hose to remove all trace of the distilled water and any deposits which also stay behind. If they remain in the cooling system and are LOOSENED by your flushing component they can do more harm clogging waterways than if they were never messed with to begin with. The waterhose method is the best way- running the waterhose pressured water through the system for 5-10min. If you can unbuckle the lower radiator hose is added extra benefit to ensure bigger pieces of contaminants are rid of. Personally I wouldn't advise driving it while the flushing agent is working- you could get stranded somewhere if an afore-mentioned bigger piece of contaminant clogs something. Idling it in park is just as good- so long as you keep an eye on your temp gauge. The better the flushing agent the cleaner and more efficient your cooling system will be.

  • 3
    How on earth can distilled water have vinegar?
    – juhist
    Commented May 10, 2018 at 7:41
  • @nudist- This forum and question is not about distillation hho gas or electrolysis. He got sound information and advice. Why don't you taste some and tell me what you think. Commented May 10, 2018 at 8:44
  • 6
    Distilled water has water and only water. That is the whole idea behind it. It does not contain gold bricks, vinegar, winning lottery tickets or anything else.
    – Hennes
    Commented May 10, 2018 at 10:50
  • Ok... Well then it does the same thing vinegar does- my mistake- the way it tastes would sure say otherwise- but fair enough. The point is it does what I said it does. Commented May 10, 2018 at 23:47
  • To improve this answer, I suggest removing the inaccurate info and adding paragraph breaks.
    – Mathieu K.
    Commented Sep 1, 2022 at 16:37

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