Most blogs or articles you find around the internet will tell you that it’s not a good idea to use water in your coolant system. There are a lot of things that can go wrong with using water as the primary fluid for cooling your engine. 

In emergency situations, however, it’s ok to use water so long as you use it only long enough to get yourself out of the emergency situation. At the first opportunity, you need to repair whatever caused your loss of coolant and refill it with the appropriate coolant as soon as possible. 

So long as the usage is short-term, you should be fine. It’s not something that you should make a habit out of, even if it’s just for short drives to and from work. Even short trips add up to one long trip as the days go on. 

Is Water Not Okay as a Coolant?

If you have mostly coolant in your system—the kind recommended by your vehicle’s manufacturer—topping it off with a little bit of water probably won’t cause much in the way of harm. 

Over time, if you keep topping it off, water will slowly dilute the remaining coolant becoming the dominant fluid in the system. Once that happens, you’ll begin having problems at some point.

• The boiling point of water
• The freezing point of water
• Internal rust/corrosion
• Engine overheating

The boiling point of water is 212°F or 100°C. Engine temperatures are more than capable of reaching the boiling point of water, which will cause the water flowing through the system to evaporate into steam. 

Coolant is typically a 50/50 combination of water and ethylene glycol. The ethylene glycol mixture raises the boiling point to around 225°F. Some specially formulated anti-freeze mixtures have an even higher boiling point, which keeps the water from turning into steam.

Once you end up with mostly water in the system, it will simply evaporate, losing the cooling effect of the water and causing your engine to overheat, as if there were nothing in there at all. The freezing point of water is 32°F or 0°C and the same applies here, but in reverse.

Water in the system can freeze if the temperatures drop below 32°F and cause all sorts of damage, especially since water contracts and expands as it freezes and thaws. Much in the same way you turn the faucets on to drip during freezing weather, the same applies here. 

Anti-freeze is also anti-corrosive and water is not. The more water, the less anti-corrosive properties you have, which will likely start a corrosive process in the internal, metal parts of the engine, which can get worse over time. 

Distilled Water

Of course, if you’re stuck out in the middle of nowhere, distilled water probably won’t be an option for you. However, distilled water is the way to go if you have to use water until you can properly fix your coolant issue and fill it back up with coolant. 

Distilled water will help prevent scaling, which is a form of corrosion, something that tap water won’t do. The recommended coolant for your system has additives in the mixture that prevents scaling and other forms of corrosion. 

If you have to use water to get you through for a few days, do your best to get ahold of a couple of gallons—more if necessary—of distilled water as your car will thank you in the long run. 

You can also get away with using purified water and soft water but you want to avoid spring water and tap water at all costs, if possible. 

How to Stop Your Engine from Overheating if There’s a Coolant Leak

Many people, who don’t have much in the way of auto mechanic knowledge, panic when their car overheats and they immediately recall every overheating horror story they’ve ever heard.

Don’t panic and certainly don’t dump any water over your engine to “cool it off.” Rapid heating and cooling in metal cause cracking. Your engine is designed to withstand the explosive energy of combustion, so it’s not likely to blow up because it got a little overheated. 

The best thing that you can do is pull the vehicle over immediately and put your hazard lights on. Unless it’s raining, get out and pop the hood. The additional circulating air will aid in the slow cooling process. 

An overheating engine is most aggravating because it means that you have to wait. Half an hour is generally how long it will take for your engine to cool down enough that you may be able to get it down the road to a gas station or repair shop before it overheats again.

While you’re waiting—as long as you are not pulled over in a dangerous area—feel free to look around the radiator and underneath to see if you can locate a leak. It may be that the fan clutch or electric fan is no longer working as well, which has nothing to do with coolant. 

You can find out if it’s the fan once the vehicle is cool enough to crank it back up. If you discover the coolant leak and you can slap something together long enough to stop the leak and get you home, feel free. 

If the leak is drastic, there’s no amount of water or coolant that’s going to do anything to help and you will have to get it towed or limp the vehicle home for what amounts to five minutes of driving for every thirty-minute break. 

A small leak means that if you can get it to a gas station, you can get the water needed to get you home. 

Final Thoughts

Water is fine for topping up your radiator in emergency situations only. If you can get away with using distilled water, and/or pure and soft water as second and third choices, that would be even better. 

The most important thing is to be safe. Nine times out of ten, when your engine overheats, you’re going to be stuck out on the side of the road somewhere. Always carry spare coolant or distilled water, just like you would a quart of oil or brake fluid. Lastly, don’t panic; it will cool down on its own in time.