Should I Use Distilled Water in My Car Radiator?

Pouring distilled water in car radiator

Should I Use Distilled Water in My Car Radiator?

Using distilled water as a coolant in your car radiator does come recommended. Distilled water is preferred over regular water, as it reduces the risk of corrosion and scaling inside the car’s engine.

Regular household water has many impurities, leading to scale formation, corrosion, and other insidious damages to the radiator.

These are the reasons why you should use the distilled water as a coolant in your car radiator.

  1. It has zero minerals or impurities that can corrode the radiator and engine parts.
  2. It has a higher heat transfer rate than most of the anti-freezing coolants available in the market.
  3. It is readily available and costs about 10% of what a regular coolant would cost.
  4. Distilled water prevents electrolysis in the car’s cooling system and prevents scale formation.

Alongside its many advantages, there are a few disadvantages to using distilled water as a coolant in your car’s radiator. In regions with very cold temperatures, the distilled water may freeze into solid ice and block the car’s engine.

Another disadvantage of using distilled water is the rust that can form inside the radiator. However, adding anti-freezing and anti-rusting coolants in a certain proportion should help eliminate this issue.

What Kind of Water do you put in a Radiator?

The best kind of water to use in a car’s radiator is softened water or distilled water. While distilled water is a more common choice, softened water is a more pure form that eliminates the risk of reaction to metal completely.

Water is changed to a gas form to leave behind all the impurities and minerals and then condensed back to its liquid state to obtain pure water during the process of distillation. During this process, water becomes void of impurities and becomes active. This leaves the water vulnerable to react with the radiator metal.

There is an ion deficiency created during the distillation process, which the distilled water tends to reduce by reacting with the radiator metal. This can lead to some serious damages leaving the cooling system stalled.

On the other hand, soft water uses salt to remove impurities and takes up the sodium ion to fill up the ion deficiency giving it the same advantages as distilled water with the reduced risk of reaction with radiator metal.

Both distilled water and soft water keep corrosion and scale formation away and have a high heat exchange rate, making them ideal for using them as coolants in a car’s radiator.


Can I Top Off Coolant With Distilled Water?

Yes, you can top off a coolant in your car’s radiator with distilled water, but without diluting the coolant mixture too much.

Often, while coolant is refilled in the radiator, there is air trapped inside it. The coolant level can drop once this trapped air escapes the radiator. To top it off, you can add distilled water along with a bit of the coolant to ensure the coolant doesn’t get overly diluted.

There aren’t many pure coolants available in today’s market. The pre-mixed coolants have distilled water mixed in them. However, if you are using pure coolants, you can continue topping it off with distilled water for a longer duration.


Can I Mix Distilled Water with Coolant?

Yes, distilled water can be mixed with a coolant. The ideal proportion for a pure coolant-distilled water mixture is 50-50.

Most of the coolants available in the market will have distilled water pre-mixed in them. Coolants have a lower heat exchange rate and hence distilled water is mixed with them to make the coolant fluid more efficient and durable.

In areas with higher temperatures, the proportion can be 30:70 or 40:60 (coolant: distilled water). Coolants have anti-freezing properties due to which they become more useful during cold weathers when distilled water may freeze.


How to Mix Coolant with Distilled Water?

Ideally, anti-freezing coolant and distilled water should be mixed exactly in a 50-50 proportion.

Mixing coolant with distilled water may sound simple. However, there are quite a few checks that need to be completed and a few steps to be followed to get it done the right way.

  1. Check Requirement

Your car’s user manual will have the specification for your coolant listed. Read the specifications thoroughly and make a note of the key factors.


  1. Buy the Specific Coolant

While one size may seem to be a proper fit for all the cars, there are certain specifications that may not match your car. Refer to the note that you’ve made based on your car’s user manual and ensure the coolant you buy is exactly the same.


  1. Mix the Coolant and Distilled Water

The coolant and distilled water mixture should be made in a separate container and not directly inside the radiator. Too much of either of the component will damage the radiator. Slightly more distilled water will not hurt your car’s engine but too much coolant will.

Distilled Water for Radiator Flush

For every 100,000 miles or 5 years, the coolant in your car’s radiator needs to be emptied and the radiator needs to be cleansed thoroughly by running water through it. This process is called Flushing.

Completely draining the radiator is not possible in most cars and hence the water used for flushing should be free of impurities to avoid scale formation and rusting in the radiator. As normal tap water might have a high amount of impurities, distilled water should be used for flushing the radiator.


How to Flush and Refill Coolant in Car Radiator?

  1. Park your Vehicle on Level

Before you start the flushing and refilling process, ensure your vehicle is parked on a level surface.

  1. Locate Radiator

Under the hood of your car, look for a metallic round cap with radiator coolant written around it.

  1. Drain the Radiator

Locate the drain valve under the vehicle and put a pan underneath. Remove the clamp and let the anti-freezing coolant drain completely.

  1. Refill with Distilled Water

Close the clamp of the drain valve and open the radiator cap. Pour distilled water until the radiator is full and close the radiator cap.

  1. Run the Vehicle

To let the distilled water settle in properly, run the vehicle for about 10-15 minutes.

  1. Prepare Coolant Mixture

In a container, mix distilled water with the anti-freezing coolant in equal proportion.

  1. Refill Radiator with the Coolant Mixture

Pour the coolant mixture into the radiator until it reaches the service line. Close the radiator cap and run the vehicle for another 15-20 minutes to let the trapped air leak out.

  1. Top Off Coolant

Once the trapped air inside the radiator comes out, the coolant level drops and needs to be topped off. Open the radiator cap and pour in the same coolant mixture that you used in the last step. Close the radiator cap.

Distilled Water Vs Tap Water for Radiator

Using distilled water as a coolant in your radiator is a better option than tap water. Distilled water is cleaner than tap water and won’t react with metal surfaces that are found in a car radiator.

Since the average time for refilling a coolant is about 5 years, it is necessary that the water used doesn’t cause corrosion or scale formation inside the radiator.

Due to the absence of impurities, distilled water becomes a more suitable option for radiators compared to normal tap water.


Scale Formation & Corrosion

Normal tap water can have impurities which can lead to scale formation and corrosion leaving the radiator damaged. Distilled water, on the other hand, does not have any impurities and has almost no chances of corrosion or scale formation inside the radiator.



One of the disadvantages of distilled water is its reactivity. Distilled water has a free ion that tends to react with the radiator metal over time. However, tap water is completely ionized and has no risk of any reactions inside the radiator.


Heat Transfer

Distilled water has a slightly higher heat transfer capacity than normal tap water, meaning that it can keep your car cool even under higher temperatures.


Cost & Availability

Distilled water is not as readily available as normal tap water and might even cost a few bucks. However, the damage tap water can cause to a car’s radiator makes distilled water the more economical option.

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