Can You Mix Coolant and Water? 5 Shocking Myths

Engine coolant, also called antifreeze, is an essential fluid for your car’s engine. It flows through the radiator and the cooling system to prevent the engine from overheating. Coolant is typically premixed with water when you purchase it. However, you may need to add more water to the coolant over time.

This raises the question – can you mix coolant and water? Or is distilled water better for diluting coolant? Let’s take a look at the pros and cons of mixing different types of water with engine coolant.

Can You Mix Coolant and Water?

Yes, you can mix coolant and water. In fact, it’s a common practice to dilute coolant with water before adding it to your vehicle’s cooling system. Most coolants are concentrated, and the recommended mixture is usually a 50:50 ratio of coolant to water.

Mixing coolant with water serves several purposes:

• It helps regulate the temperature of the engine.
• It prevents freezing in cold weather.
• It protects against overheating in hot conditions.
• It provides corrosion and rust protection for the engine’s components.

How Much Water is to be Mixed in the Coolant?

The ratio of coolant to water can vary based on the manufacturer’s recommendations. Typically, a 50/50 mix of water and coolant is ideal for most engines. This means equal parts antifreeze and water. This ratio provides maximum heat transfer capabilities while preventing corrosion and freezing. 

Some vehicles require more or less water, so always check your owner’s manual. When adding fluid, it’s best to add premixed coolant instead of straight water to maintain the proper ratio.

What Type of Water to Mix with Coolant?

Since tap water can cause problems, use distilled or deionized water when topping off coolant. Here’s what makes them better choices:

• Distilled Water – This is purified water that has been boiled and condensed to remove minerals. It will not corrode system components or leave mineral deposits. Distilled water mixes well with the antifreeze and won’t dilute anti-corrosion additives.
  
• Deionized Water – This is water that has passed through a special filtration process to remove calcium, magnesium, and other minerals. Like distilled water, it will not corrode the cooling system when mixed with coolant.
  
• Softened Water – While better than tap water, softened water still contains some minerals like sodium. It’s okay for an occasional top-off but frequent use can still corrode over time. Distilled or deionized water is a better option.
  

Using bottled drinking water is not recommended. It often comes from a municipal source and contains minerals. Unless specified as distilled or deionized, bottled waters can still contribute to cooling system corrosion.

Which is best: Distilled or deionized water for coolant

Deionized water is even better than distilled because it undergoes additional purification to remove all ions making it incredibly pure. The purity of distilled and deionized water means they have better heat transfer properties to act as an effective coolant without leaving residues. This protects engines and mechanical systems better than tap water-based coolants.

Advantages of Using Distilled Water with Coolant

Here are some key benefits of using distilled or deionized water to top off your coolant:

• Eliminates mineral deposits that can clog coolant passages
• Won’t dilute corrosion inhibitors like tap water
• Maintains optimal boiling point for heat transfer
• It helps prevent electrolysis damage to engine components
• Works safely with all types of coolant
• Resists freezing better than water alone

Distilled water is compatible with any color coolant – green, yellow, red, or orange. It will extend the life of your coolant while keeping the freeze protection, cooling, and anti-corrosion abilities intact. Just remember to limit water top-offs to maintain the ideal 50/50 mix of water and coolant concentration.

Where to Buy Distilled Water for Coolant

It’s easy to find distilled water for your cooling system:

• Grocery stores – Sold in gallon jugs, often in the water or pharmacy section
• Convenience stores – Look in the beverage section
• Auto parts stores – Sold in gallon containers in the chemicals/fluids section
• Hardware stores – Check the paint or cleaning product aisles
• Gas stations – Some stations have gallon distilled water jugs
• Online – Amazon and other retailers sell distilled water for delivery

One gallon is often sufficient to top off coolant when needed. Walmart, Target, and auto parts stores also sell premixed 50/50 coolant if you need more than just water.

Steps to Mixing Coolant and Water

When topping off your coolant, follow these steps:

1. Locate the coolant reservoir tank under the hood. It will have “coolant only” or a similar warning on the cap.
   
2. Remove the reservoir cap once the engine has fully cooled. Wear hand protection to avoid burns.
   
3. Add the recommended mixture of coolant and distilled water. For most vehicles, this is a 50/50 ratio.
   
4. Mix by starting the engine and letting it run for a few minutes. This will circulate the fluid.
   
5. Replace the reservoir cap securely.
   
6. Check for leaks and proper coolant level over the next few drives. Top off if needed.
   

Mixing distilled water with premixed coolant is always preferred. But this method can add small amounts of water safely in a pinch. Let the engine fully cool before removing the cap to avoid burns.

Proper Ratios for Mixing Coolant and Water

Follow your vehicle manufacturer’s recommendations for coolant-to-water ratios. Here are some general guidelines:

• For most modern vehicles, use a 50/50 mix of coolant and distilled water for optimal performance. This ratio protects down to 34°F.
  
• For heavy-duty or high-performance engines, a 60/40 mix (60% coolant, 40% water) provides protection down to -62°F.
  
• In very cold climates, a 70/30 mix (70% coolant, 30% water) safeguards down to -84°F.
  
• Adding more than 70% coolant can harm heat transfer and offers little added freeze protection.
  
• Don’t use less than 40% or more than 60% water depending on climate. Too much water reduces corrosion inhibitors.
  

Checking your owner’s manual will give the exact ratios required for your vehicle.

Mixing Coolant and Distilled Water in the Radiator

You should not remove the car radiator cap to top off the coolant and water mixture. Only add fluid to the coolant reservoir. Removing the pressurized radiator cap can be dangerous and should only be done by a mechanic during a drain and refill.

The coolant reservoir tank acts as an overflow tank. When the engine heats up and the coolant expands, the excess flows through the radiator cap down into the reservoir. Adding distilled water or premixed coolant here allows it to mix properly when pressure pushes it back into the radiator as temperatures change.

Only open the radiator cap once the engine has fully cooled. Even then, use extreme caution when topping off directly in the radiator. Turning the cap slowly releases built-up pressure that could spray hot coolant. Consult your owner’s manual for the proper procedures to top up the radiator directly if needed.

Read More: 7 Symptoms of Low Coolant in Car: Warning Signs

Potential Issues of Mixing Coolant and Water

There are a few risks to be aware of when adding water to your coolant:

• Using tap water can introduce minerals and contaminants that lead to corrosion and deposit buildup in the radiator, water pump, and other cooling system components. This can reduce heat transfer efficiency.
  
• Tap water has a lower boiling point than a 50/50 coolant mixture. This reduces the coolant’s ability to prevent overheating at high engine temperatures.
  
• If you use too much water, it can dilute the coolant and reduce its freeze protection during cold weather. This can lead to cracked engine blocks, radiators, and hoses.
  

To avoid these issues, it’s best to use distilled or deionized water when topping off your coolant. Let’s look closer at the effects of mixing regular tap water with engine coolant.

What Happens if You Mix Tap Water with Engine Coolant?

Tap water contains minerals and chemicals like calcium, magnesium, chlorine, and fluoride. If used over time, these minerals can corrode the aluminum and other metals in your engine and leave mineral deposits that clog small passages in the radiator, water pump, and cylinder heads.

The minerals in tap water also raise its boiling point above that of a 50/50 coolant mixture. This reduces the coolant’s ability to transfer heat effectively and maintain safe engine temperatures.

In most cases, a one-time top-off with tap water won’t immediately damage the engine. However frequent use or using a larger ratio of tap water will dilute the corrosion inhibitors in the coolant. Without enough coolant, the metals in the engine and cooling system will start to corrode and degrade over time leading to coolant leaks and overheating issues.

The bottom line – don’t use tap water as your go-to source when topping off your coolant. Only add premixed coolant or distilled water.

Accidentally Mixing Coolant and Water

What if you’ve already accidentally mixed tap water with your coolant? Don’t panic. The best course of action is to drain out the old coolant and water mixture at your next fluid change. Refill with fresh premixed coolant to reset the 50/50 ratio. This will dilute any minerals and replenish the anticorrosion additives.

If a significant amount of tap water is added, you may want to get the cooling system flushed by a professional mechanic. They can ensure there is no scale buildup in the narrow coolant passages.

Going forward, only top up with premixed coolant or distilled water to avoid diluting the mix. Check your coolant level and condition more frequently when adding any amount of water.

Can you use water instead of coolant in the summer?

No, it is not recommended to use plain water instead of a proper coolant mixture even in hot summer weather. Here are a few key reasons why:

• Coolant has additives that protect against corrosion and rust. Water alone could allow metal parts to corrode over time from the inside out.
  
• Coolant lowers the freezing point well below 32 degrees Fahrenheit. Using just water risks freezing and damaging the radiator or engine in cooler weather.
  
• Coolant acts as an anti-scale agent to prevent mineral buildup from hard water. Deposits could eventually clog radiators, hoses, or small passages.
  
• The coolant mixture has a higher boiling point than water alone. Overheating is possible on hot days if the boiling point is too low.
  
• Coolant lubricates water pumps and seals to prevent leakage. Water may not provide the same lubricating protection.
  

While the cooling demands are lower in summer, proper coolant is still needed to protect the cooling system and prevent overheating or freezing. It’s best not to replace coolant with plain water even if temperatures are high.

FAQs

What happens if you don’t dilute coolant?

If coolant is not diluted as directed, it can lead to several issues within the cooling system. Undiluted coolant becomes too viscous and thick, hindering proper circulation and heat transfer and leading to engine overheating.

Over time, the high concentration of additives may also result in accelerated corrosion of seals, hoses, and other components. Dilution as specified ensures optimum protection and system functionality for the lifetime of the vehicle.

Can you use undiluted coolant?

No, it’s not safe to use undiluted coolant. Undiluted coolant is too thick and viscous, which can restrict flow through narrow passages in the radiator, water pump, and elsewhere in the cooling system. This restricted flow risks overheating issues due to inadequate heat transfer.

Conclusion

It is crucial for both engine and cooling system protection to dilute coolant concentrate as recommended by manufacturers. Using undiluted coolant can strain components, restrict vital flow, and potentially negate the additives put in place to prevent freeze, boil, and corrosion damage over time.

Proper 50/50 dilution strikes the right balance for heat transfer ability and long-lasting protection under all operating conditions. While it may seem like a minor step, taking the time to measure and blend coolant concentrate according to guidelines pays off in extended equipment life, trouble-free operation, and lower repair costs down the road.