Engine Coolant mixed with tap water

I have inadvertently added a little tap water into my coolant reservoir. Car is running fine. Do I need to worry about anything now, or do I need to bleed the system just yet?


Water is what you mix with coolant to make a 50/50 mix of anti-freeze and water. Usually distilled water but a small amount of tap water will do no harm. No you don’t have to bleed the system.

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Thanks. It’s my first car so was not sure.

How much is a little?

About a 100ml at most. But not sure it’s up to

That is very little. If you have to add coolant regularly you may have a leak. It doesn’t appear you do. Just check your levels Every couple of weeks. You could get a coolant tester. They’re cheap to test antifreeze concentration. Then look up the maintenance schedule and change coolant when it’s due.


100 ml added to the overflow tank shouldn’t cause problems. However, before freezing temperatures arrive, make sure to have the coolant tested for proper freeze protection.

In more than 60 years of driving I have never mixed my coolant with anything but tap water.

I suppose it could depend on where you live. Our water is not very "hard, nobody I know uses a water softener. My water comes from the Niagara river.


I would not do anything right now to the cooling system. 100ml of tap water isn’t going to harm anything, but I would not make a practice of using tap water. Eventually you could get enough minerals to do some damage.

I suggest that you get a gallon of premix for your vehicle. Most “universal” coolants are OK for top off but if that makes you anxious, then go to the dealer and get a bottle from them, it doesn’t cost that much more. Peace of mind has value too.

For longest life, follow the owners manual recommendations for coolant changes, just drain and fill and NEVER flush and most important, NEVER EVER use a chemical flush. (edit: in a working system)


and bleed the air out of the system.


Best to use distilled I guess but for 50 years I just used tap water and never had a problem.