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Distilled water for radiators?

This spring I disassembled an engine from a 1965 MGB that has been in the family for the last 30 years. It has always been filled with a 50/50 mixture of Prestone and tap water. There was virtually no rust anywhere in the cooling system. Now I am hearing that you should only use distilled or de-ionized water. Based on my experience, it would be a needless complication, although rather minor in cost and nuisance. By the way, as far as I know, the MG still has the original radiator.

Also, if you go to the Prestone web site, they show a radiator being filled by a garden hose.

Is there any factual information that would cause one to believe that the use of tap water is harmful?


  • edited April 2008
    I would imagine that it depends on your tap water, and the amount of dissolved "stuff" in it. Water quality varies a lot across the country. If you have very hard water, I suppose its possible that mineral deposits could develop and potentially clog the small conduits in a radiator, but it depends on if the chemistry of coolant, and whatever else.. the metal its in contact with, would allow for that to happen. Any water chemists out there want to elaborate?

  • edited April 2008
    It doesn't matter what kind of water is mixed with the coolant. Be this tap water, distilled water, de-ionized water, bottled water, dog water, etc..... As long as the cooling system is maintained on a regular basis, the antifreeze at the proper mixture will prevent any corrosion from ocurring.

  • edited April 2008
    I believe it depends on the quality (hardness) of your tap water. Personally, I only used distilled water in my cars, it is very cheap and available everyplace.
  • edited April 2008
    In most areas there is no problem using tap water. There are some places where the water is not so good, usually on the acid side and if it overcomes the buffers in the coolant, there can be some problems so using distilled water will not hurt. That said, I have never bothered and I would guess few people do and very few experience any problems by using tap water. It is important to change the coolant as it appears you have been doing. If it has worked for the last 40+ years, I suggest keep doing what you have been doing.
  • edited April 2008
    What they said. If the tap water is hard, it might be marginally better to use distilled water to fill. I should still be O.K., even, to flush with tap water. You will see little problem with water quality in a closed system like this compared to a chilling tower, steam boiler or water heater where the water is constantly being changed. There, corrosion and scaling are huge (expensive) issues.

    Even acidic water should not be a problem as it will not have significant buffering capacity compared to fresh coolant. I would not use dog water, use nothing yellow in there unless it comes out of a coolant jug.
  • edited April 2008
    De-ionized water has no minerals in it, and there for is a sponge looking to absorb minerals, like the radiator, and other metals in the cooling system, Not what I think you would want. A matter of fact, plumbing for Di-ionized water needs to be stainless steel or plastic.
  • edited April 2008
    It's not a problem when mixed with the correct antifreeze in your cooling system. The intent of using distilled water is to avoid minerals from the water being deposited in the cooling system.
  • edited April 2008
    distilled and de-ionized water is not the same. Although after doing some research I did read were Di-ionized water is preferable.
  • edited April 2008
    I would just buy pre-mixed antifreeze/coolant. That would easily resolve the issue. Tap water is fine though, as long as you change it on schedule. Don't give the antifreeze/coolant chemicals a chance to stop working. Drain and refill every two years.
  • edited April 2008
    That is correct, I use distilled water in cooling systems. Even de-ionized water should be harmless when mixed with antifreeze containing corrosion inhibitor packages. I certainly would not use straight de-ionized water in a cooling system.
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