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Checked my coolant level today and noticed in the reservoir it was lower than the “Low” line so I added 100% antifreeze which brought it up between the “L” and “H” marks as I couldn’t see properly

Questions:

  1. Will adding this little amount to a 50/50 mixture already in the car do any harm? If so should I add some water to the reservoir?

  2. What type of water should be added, distilled or de-ionized? I’ve read very conflicting info about each with lots of people confusing the two for the same thing

  3. Does pure 100% coolant stored in an opened but tightly capped container last forever? I’ve had one for about 10 years that I still use. Color looks the same since day one

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  • How much antifreeze did you add, and how big is the reservoir? – GdD Dec 19 '17 at 16:23
  • I added probably 1/2 of a cup. It’s a 98 civic if that helps with reservoir size – ohmmy Dec 19 '17 at 16:25
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    1/3 of a cup? Fuggedaboutit, that's not enough to worry about @ohmmy. – GdD Dec 19 '17 at 16:27
  • What about the other questions? And how much is enough to worry about it lol for future reference – ohmmy Dec 19 '17 at 16:28
  • Questions 2 and 3 have already been answered: mechanics.stackexchange.com/questions/11972/… – GdD Dec 19 '17 at 16:47
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  1. Will adding this little amount to a 50/50 mixture already in the car do any harm? If so should I add some water to the reservoir?

If the cooling system is at a true 50/50, adding a little more coolant will not cause an issue. 50/50 is a good mix, but the system should do just fine up to a 70% solution of antifreeze over water. In the future, using from a premix bottle of 50/50 will preclude this issue.

  1. What type of water should be added, distilled or de-ionized? I’ve read very conflicting info about each with lots of people confusing the two for the same thing?

Distilled should be used. De-ionized water will still have impurities in it, which is what you want to avoid. Again, using coolant out of a premix jug will save you the trouble.

  1. Does pure 100% coolant stored in an opened but tightly capped container last forever? I’ve had one for about 10 years that I still use. Color looks the same since day one

It is my understanding, as long as the jug is closed up, it's usable. When you turn antifreeze into the recycling place, they basically just filter it through really well, reseal it in a container, and put it back out for sale. In fact this website states:

Is used antifreeze as good as new antifreeze?

In a word: yes. The American Society for Testing and Materials and the Society of Automotive Engineers have done repeated studies on recycled antifreeze and determined it performs just as well as the new product. In fact, the EPA posits that secondhand antifreeze is actually better because the recycling process removes chloride often found in new antifreeze.

Why would that be important to this conversation? Because if the antifreeze doesn't have any of the impurities you'd find through use, it should still be good because there's nothing there to make it bad. A closed container should keep antifreeze in good shape indefinitely.

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  • My understanding is de-ionized is cleaner than distilled? I read conflicting info about de-ionized stealing ions from the metal and shouldn’t be used but then I read it’s fine to use so idk – ohmmy Dec 19 '17 at 17:05
  • @ohmmy - Distilled is just pure water. De-ionized water still has all of the impurities (like heavy metals/chlorine) in it which can be hard on your engine. – Pᴀᴜʟsᴛᴇʀ2♦ Dec 19 '17 at 17:07
  • @ohmmy - Disregard my last about what de-ionized water is, because I was wrong. While I was wrong about what it is, the fact is, you still want to use distilled water instead of de-ionized water. As this article states, if you leave de-ionized water out to mix with air, which came from distilled water, it will then become distilled water. Distilled water does remove most, if not all, of the impurities you don't want in your cooling system. De-ionized water is just an added expense which isn't needed. – Pᴀᴜʟsᴛᴇʀ2♦ Dec 19 '17 at 17:19
  • Thank you that extra info helps. So deonized water is usually distilled water first that is then removed of ions? – ohmmy Dec 19 '17 at 18:36
  • @ohmmy - Distilled water is one starting point. As the article points out, it can be made from tap, spring, or distilled water. – Pᴀᴜʟsᴛᴇʀ2♦ Dec 19 '17 at 18:51
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As for question 1, your Civic takes between 4 and 4.5 liters of coolant, that is about 20 cups. 1/2 a cup is therefore about 1/40th of the total of your coolant, that's not enough to throw things out, but if you're concerned get a tested.

Regarding deionized or distilled water, deionized is the purer of the two as it has all mineral material removed, distillation is pretty pure but some organic materials will boil off with the water and therefore remain. However, for the purposes of coolant either is fine. If you have soft tap water then that would be fine as well.

Question 3 is answered already in another question, the accepted answer is:

The coolant should not expire any time soon. If it is not in an automobile being used, it is not degrading. With the cap on it, there is nothing contaminating it.

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  • Thanks for your response. Regarding de-ionized water I’ve read info saying since all ions are stripped from it then it can steal some from the metal in and engine/radiator? Others say it won’t matter even if it did so this is why it’s confusing – ohmmy Dec 19 '17 at 17:07
  • De-ionized water isn't going to steal anything, anti-freeze has all sorts of additives to inhibit corrosion @ohmmy – GdD Dec 19 '17 at 17:27
  • I'm sorry, @GdD, I'm not sure how you can claim that? De-ionized water, when it sits in air for any amount of time, reacts with the CO2 in the air and will become acidic (up to about 5.6 on the pH scale), which means it will attack metals. While not a strong acid, is still acidic in nature. Please read this page for clarity. – Pᴀᴜʟsᴛᴇʀ2♦ Dec 19 '17 at 18:43
  • I'm not saying it's non-reactive @Paulster2, I'm saying that when mixed in with anti-freeze it's a non-issue. – GdD Dec 20 '17 at 8:38

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