The sun reflects through a water bottle held in a person's hand.

Can I Put Water in My Car Radiator?

You’re driving when you notice the temperature gauge start to creep up. You pull over, safely check the coolant overflow reservoir and find that it’s bone dry. You’re not carrying coolant, but you do have water.

You might be asking yourself: “Can I put water in my car radiator?”

Yes, you can. And in that situation, you should. Water in your car radiator beats nothing, hands-down.

Water Versus Coolant

A long time ago — a really long time ago — cars ran with water in their radiators most of the time and people added antifreeze when it got cold. Back then, cars were simpler, engines ran cooler and, unless something was wrong, the boiling point of water (212 degrees Fahrenheit at sea level) was high enough that it worked just fine. Water is ejected from a bottle.

Modern engines operate at higher temperatures. Running just water in your car’s radiator will guarantee overheating and damage, including to your cylinder heads and engine block. And most tap water contains minerals that will leave deposits inside the radiator, causing corrosion, shortening its life and further diminishing its ability to cool.

Coolant provides a protective coating for your radiator to inhibit corrosion. A 50/50 mix of coolant and water also lowers the freezing temperature and raises the boiling point of the water in your car’s radiator. Instead of freezing and cracking your engine block at 32 degrees Fahrenheit, a half-water, half-coolant mix has a freezing point of negative 34 degrees. The boiling point takes a healthy jump from 212 degrees to 265, as well.

So, if you find yourself in the situation we started with — asking yourself if you can put water in my car radiator — make sure it’s for a short distance only — just far enough to buy and add coolant. Then, see a mechanic to find out why you were low in the first place, repair any leaks and flush the system, and add a fresh 50/50 blend of water and coolant with a funnel.

Why Not Use Just Coolant?

That’s too much of a good thing. Remember, water has natural cooling properties. It’s why you drink it. The coolant’s job, beyond inhibiting corrosion, is stretching those properties to make freezing or boiling happen outside the normal range of temperatures your engine experiences. Coolant by itself would freeze at 8 degrees Fahrenheit. Wouldn’t you rather be protected to negative 34?

Check out all the heating and cooling products available on NAPA Online or trust one of our 16,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on radiator failures, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Flickr.

about author

Mike Hagerty

Michael Hagerty is an automotive and travel writer whose work has been featured on radio, TV, in print and online since 1997.  Outlets have included the ABC television affiliates and Hearst-Argyle and Emmis radio stations in Phoenix, Arizona;  AAA magazines for Arizona, Oklahoma, Northwest Ohio, South Dakota and the Mountain West, BBC.com and Mr. Hagerty's own website, TireKicker (one of the few automotive sites to have a nearly equal split between male and female readers).  Hagerty brings straight talk and perspective to his writing, blended with a tight, upbeat style.  He is also a prolific writer trained to perform under tight deadlines.  Quick turnarounds are his specialty.  Located in Folsom, CA, Hagerty is only two hours from the Bay Area and a half-day's drive from Los Angeles.  Adept at social media, Hagerty's Google Plus page boasts 4.5 million views and counting.

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