Shadetree machanic

i know when you are mixing coolant you use distilled water.but will tap water do any damage.

Most of the time no. However water is not water. Tap water varies a great deal from place to place. If it has something in it that is harmless to humans it may not be harmless to your car. What does the coolant container say on this subject?

I’ve Always Imagined That If You’ve Got Traces Of “Stuff” In Your Tap Water It Can Cause Scale Or Sedimentation In Your Cooling System, Especially When Heated And Cooled Repeatedly.

Do you have hard (dissolved limestone from well) water ? Salt from a softener ?

I always go distilled for that reason. I make my own, but it’s cheap enough to buy.


Adding tap water to a cooling system will do no harm. Water hardness is measured in grains per gallon or parts per million. This small amount of minerials in the water will have no effect on the cooling system. If the system were one where tap water was constantly running thru it, then the possibilty exists where minerial deposits can accumulate.


From Peak Brand Antifreeze Website:

Q: Can I mix ordinary tap water with antifreeze?

A: Yes, tap water is commonly used for mixing with antifreeze, however, deionized water or distilled water is preferred. Do not use water softened with salts to mix with antifreeze.

I hold with my advice about softened tap water (and distilled water, too).


I never was informed by any manufactures rep. not to use tap water. BMW includes coolant “drop and fills” in it’s maintence schedule and every BMW shop I have ever been in reflects this. What I mean is there always seems to be coolant bottles everywhere and always coolant on the floor from so much work with the coolant. I questioned the pratice of letting this spilled coolant get into the sewer system in one Milwauke Dealership and was told that the county did not view the amount of coolant we were introducing into the sewer system as an issue, I did find this rather odd but I asked and got an answer.

Our water has 25 grains per gallon of hardness. I have been using it prior to being softened for many years in our vehicles with no problems including in one vehicle over 20 years old with the original radiator. Possibly some water is not as good for this use so any mfr’s warning would cover for any tap water.

Old School: Milwaukee water should be fine for radiators as it is very soft as is, coming from Lake Michigan. People using Milwaukee water do not need water softeners.