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Do Cars Need Water to Run Properly?

A mechanic pouring in water from a water bottle into the radiator with the radiator cap removed

When people talk about “topping off fluids” in a car, what does that entail? You might have seen someone pouring a water bottle into their engine on the side of the road — do cars need water to run or is this just a quirk for certain cars?

Cars don’t specifically need water to run properly. Water is part of coolant, which is needed, but water on its own should not be used unless there’s an emergency and your car is overheating. Coolant uses a mixture of water and additives that change the boiling point and freezing point of the water. This coolant circulates around your car, keeping your engine’s temperature low.

In this guide, I’ll be talking about water and its role in your car. I’ll also give a crash course on coolant, why it’s so important, and how it works.

Understanding Overheating

I know that you’re probably asking about using water as a means to keep your car cool. There’s a system in your car that helps keep your engine cool because overheating is a big problem.

Overheating happens when an engine gets too hot. There’s a lot of motion going on under the hood in addition to small explosions happening in your engine — both of which generate a lot of heat. When things get too hot, your engine can seize, and things can start bursting.

Car overheating radiator blowing out hot smoke

In extreme cases, pistons can weld themselves to the cylinders in your engine, potentially totaling your car.

To put it simply, overheating is a big and expensive problem. That’s why your car has a coolant system, to begin with.

How the Engine Coolant System Works

Before talking about how water can help, I should explain the coolant system in your car. As I mentioned, this is included just to keep your engine’s temperature down, but how does the coolant system work?

Coolant is a chemical that’s very good at absorbing energy and slowly changing from a liquid to gas and back. In your car, it’s used to usher heat away from sensitive areas of your car — more specifically, the engine.

Engine Cooling System Illustration

Your whole coolant system relies on a water pump that moves around the liquid in the lines. If the liquid becomes gas, it won’t flow correctly. Instead, gas will occupy the lines near the hot areas of your car, and the liquid will get stuck.

The coolant enters your engine block and head, then travels across the block, absorbing heat as it goes. Doing this will make the engine cooler and heat up the coolant.

The hot coolant will travel to the radiator. A radiator is just a way to cool down the coolant and prepare it for another loop across the engine. The radiator has loops of metal tubing that expose the coolant to incoming cold air. The air makes the radiator cooler and brings it back to a liquid before it gets pumped back to the engine and the loop continues.

What is Engine Coolant?

Coolant is a chemical mixture of water and some additives. The only purpose of the additives is to shift the boiling and freezing point of the coolant.

In reality, it’s close to a 50/50 mixture of water and additives. The fact that coolant uses so much water is a good hint that water on its own can also help.

What Does the Water Do?

When people add water, they’ll typically do so in an emergency situation. When your car is overheating and you pull over to deal with it, that’s when the water comes out.

People will pour water into their coolant reservoir or directly into the radiator. There are two different caps that need to be removed to add water. Even lukewarm water is hundreds of degrees colder than your engine, so adding water will cool everything down momentarily.

In the long run. water on its own should never be poured into the coolant line of your car unless it’s an emergency. If your car is just using water, a few problems are going to occur.

A person filling up the engine coolant reservoir with water to help with the engine overheating

First off, the water can freeze in the lines since there are no additives in the water…this happens when the vehicle is in a cold environment and gets under 32 degrees F. If you’re parking outside, this might commonly happen during winter. Due to that fact, your engine can quickly overheat and seize.

The other problem with water is that it will boil too soon. As I mentioned, coolant is used to absorb heat from the engine and move it away from the hot area. Since water boils too soon, it simply becomes a gas and moves away without absorbing enough heat.

If you don’t have any coolant in your coolant lines, you’re asking for trouble. When water becomes a vapor, it can escape the coolant system which means you don’t have enough liquid to reliably keep your car cool.

If you rely on water alone, you’ll be pulled over frequently, adding more water as your car overheats.

Water vs Coolant: Which Should You Use?

In general, you should rely on coolant to keep your engine at a good temperature. As I mentioned, coolant is specifically engineered to absorb heat and keep it moving.

Water should only be used in an emergency situation. If you notice your engine temperature is getting dangerously high and you don’t have coolant on you, then water might be your only option.

It works in a situation like this because the water is cooler than your engine, so it will keep the temperature down. It’s more of a temporary fix because your engine will overheat again pretty quickly since the water will just evaporate.

A mechanic pouring engine coolant into the engine coolant reservoir green fluid with gloves on

If you notice a nearby auto shop while your car is overheating, it would be best to go there and pick up coolant. Refilling your car with coolant instead of water will give you better long-term results as you fix the problem with your car.

I should mention that water shouldn’t be your solution to an overheating car. The real solution is to figure out why your car is overheating and fix it. The problem could potentially only take a few minutes and dollars for a permanent fix.

The water is used in this case just to get your engine temperature down for a little bit. Overheating engines can weld engine parts and total your car, so getting the temperature down is critically important.

Do Cars Need Water to Run Properly?

To specifically answer the question, cars don’t necessarily need water to run properly. They need coolant to run properly, which is comprised of water, but water on its own shouldn’t be used long-term.

Coolant is cycled through your engine to draw heat away from the block and keep the operating temperatures lower. Your engine has a specific range of temperatures that it likes to run at. If it gets too cold or too hot, the performance will suffer, and you can experience some massive engine damage.

 If there’s a coolant leak, then your car will need water in order to run properly. If everything is fine, then coolant alone should be used to keep your car running properly.

The Right Way to Add Water or Coolant

Let’s say you’re on the side of the road with an engine that’s billowing steam — what are you supposed to do?

Confirm It’s Overheating

Before doing anything else, you want to confirm that your car’s problem is that it’s overheating. The best way to do that is to check your dashboard for your engine’s temperature gauge.

Car temperature gauge overheating 2
Car temperature gauge overheating

It has a range from “C” to “H” and a dial that should point somewhere in the middle. If the dial is pointing at the “H” or beyond, then you have an overheating problem.

This step is important because it will ensure you’re fixing the right problem. If you have a blown gasket, then adding water won’t fix any problems.

Check the Coolant Level

It’s also a good idea to check the level of coolant in your reservoir. Remember, the coolant system is a closed-loop that continually recirculates the same supply of coolant. The coolant is held in the reservoir until the pump brings it out and circulates it through the engine.

If your reservoir is empty, then there’s no coolant to recirculate which means heat isn’t getting taken away from the engine.

Close up of an engine coolant reservoir with pink coolant fluids and side markings to inspect coolant levels

Typically, the reservoir is in the corner of the engine bay, near the engine block. You can usually spot it easily since it’s a semi-transparent plastic bin with a bright pink fluid inside of it. That pink fluid, of course, is your coolant.

There should be a line on the side of the tank that indicates how much fluid you should have. If you have anything less than that, it’s time to add coolant.

Find the Radiator and Pour in the Water

If your car is overheating and you’re low (or out) of coolant, the first place to add water is at the radiator. Before you do anything, you need to understand how unsafe this can be. The radiator could be filled with scalding-hot, pressurized fluid. Simply grabbing and loosening the cap can seriously burn and injure you.

I suggest grabbing a thick towel and very slowly loosening the cap. You want to be very slow, and when you hear the pressure start to release, stop unscrewing and back up for a minute or two. This will give your radiator time to depressurize before you blow off the cap.

Once it’s done, unscrew the cap further. If you don’t hear any more pressure escaping, then fully remove the cap and put it somewhere nearby on a metal component.

Close up of the car engine coolant radiator cap with a warning not to open while hot

Now, your radiator is exposed. You’ll notice an indicator in the tube leading to the radiator, this is the “max fill line” — as you’re pouring in water, keep a close eye on this line, you don’t want to overfill it.

This will help cool your engine down until you get home and can officially fix the problem.

You can also add water directly to the coolant reservoir you spotted earlier, and this will further help keep the engine cool.

You shouldn’t drive away until your engine temperature is in the middle of the temperature gauge on your dashboard (not near the “H”).


As you just learned, your car doesn’t need water alone to run properly. It can be used if your car is overheating and you don’t have access to coolant, but your car will always prefer coolant as a permanent solution. If you want to learn more about cars and what they need, take a look at my blog. I also have a list of car products that I highly recommend, so check that out.

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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