Does the car need to be running when adding coolant?

No. In most cases you should add coolant only when the engine is cool and the car is off, to avoid burns and a pressurized spray of hot liquid.

Coolant systems operate under pressure, and opening a hot radiator or reservoir cap can cause severe scalding. While some vehicles allow a brief run after topping up to help circulate the coolant and bleed air, this should only occur after the engine has cooled and the cap is securely in place. Always consult your vehicle’s owner manual for model-specific guidance.

When and how to top up coolant

Following standard precautions helps you top up safely and avoid damage to the cooling system.

Follow these steps to safely add coolant.

1. Park on a level surface and switch off the engine. Let it cool completely before touching any part of the cooling system; never attempt this with a hot engine.


2. Locate the coolant reservoir and check your owner's manual for the correct coolant specification and mixing ratio (50/50 premixed or concentrate that must be diluted with distilled water).


3. With the engine cool, slowly remove the reservoir cap to relieve any residual pressure, then remove it completely if your manual allows.


4. Check the current coolant level; if needed, pour in the recommended coolant (or premixed solution) until you reach the MAX fill line. Do not overfill.


5. Secure the reservoir cap firmly, ensuring a proper seal.


6. Start the engine and run it for several minutes with the heater on to help circulate coolant and bleed any trapped air, watching for leaks and for the level to settle.


7. Turn off the engine and let it cool again, then recheck the level. Top off if necessary and replace the cap.

After topping up, monitor the system for leaks and check the level again when the engine is cold. If the level continues to drop or the temperature gauge climbs, stop and seek professional service, as there may be a leak or another issue.

Common mistakes to avoid

These are common missteps that can lead to cooling system problems or safety hazards.

• Opening the radiator or reservoir cap while the engine is hot, which can cause a violent spray of hot coolant or burns.


• Using the wrong type of coolant or incorrect dilution for your vehicle, which can cause corrosion or poor cooling performance.


• Overfilling the reservoir, which can lead to spills and pressure issues.


• Forgetting to bleed air from the system if your car requires it, potentially causing overheating or air locks.


• Using tap water in vehicles that require distilled water or a specific coolant mix, which can introduce minerals and scale.

By avoiding these mistakes, you reduce the risk of overheating and coolant leaks and help extend the life of the cooling system. If in doubt, consult the owner’s manual or a qualified mechanic.

Vehicle-specific notes

Some modern cars have sealed cooling systems or require specialized bleed procedures. Always follow the guidance in your owner’s manual or contact the manufacturer for model-specific instructions.

Summary

The short answer is that you should not be running the engine when you add coolant. Wait for the engine to cool, use the correct coolant formulation, and top up carefully up to the recommended level. Afterward, run the engine briefly to circulate the coolant if advised by your manual, then recheck the level once cooled. If you notice overheating, leaks, or uncertainty about the procedure, seek professional help.

What is the proper way to add coolant to a car?

To add coolant, first ensure your car's engine is cool and then find the coolant reservoir or radiator cap, which are usually marked with a radiator symbol. Slowly loosen the cap to release any pressure, then use a funnel to add the correct type of coolant or a 50/50 mix of coolant and distilled water up to the "cold fill" or "max" line on the reservoir. After closing the cap, start the engine to let the coolant circulate and then re-check the level once the engine is cool and top off if necessary.
 
Step-by-step guide
This video demonstrates how to add coolant to your car's coolant reservoir: 1m2CarProsYouTube · Oct 6, 2016

1. Wait for the engine to cool. Never open the cooling system when the engine is hot, as the system is under pressure and hot fluid can spray out and cause burns. 
2. Locate the coolant reservoir or radiator cap. The reservoir is a plastic tank with "min" and "max" lines, while the radiator cap is on the radiator itself. 
3. Loosen the cap slowly. To release any residual pressure, turn the cap a small amount, step back, and listen for a hissing sound before removing it completely. 
4. Add the correct coolant. Check your owner's manual to confirm the correct type of coolant or if you can use a 50/50 mix of concentrated coolant and distilled water. 
5. Pour the coolant. Use a funnel to pour the liquid into the reservoir until it reaches the "cold fill" or "max" line. Do not overfill. 
6. Replace the cap. Securely tighten the cap back on the reservoir or radiator. 
7. Start the engine. Let the car idle and monitor the temperature gauge to ensure it reaches normal operating temperature and no leaks appear. The coolant level may drop as the engine heats up and the thermostat opens. 
8. Top off if necessary. Once the engine is cool again, check the level and add more coolant to the fill line if needed. 

This video shows how to fill the coolant reservoir to the correct level: 48sDIY with Michael BordersYouTube · Oct 13, 2018

Can I add coolant while the engine is hot?

No, you should never add coolant to a hot engine due to the severe risk of serious burns from scalding steam and the potential for damaging engine components. The cooling system is under high pressure when hot, and opening it can cause boiling fluid to spray out, so you must always wait for the engine to cool down completely. 
Dangers of adding coolant to a hot engine

• Severe burns: Opening the system while it's hot can release high-pressure steam and boiling liquid, which can cause scalding and serious burns. 
• Engine damage: A sudden temperature change from adding cold liquid can cause parts like the cylinder head or block to warp or crack. 

How to safely add coolant

1. Shut off the engine: and wait for it to cool down completely. This can take at least 30 to 60 minutes, depending on the engine. 
2. Locate the coolant reservoir, which is usually a translucent plastic tank connected to the radiator. 
3. Check the fluid level: against the "hot" and "cold" lines on the reservoir. The level will be higher when hot and lower when cold. 
4. Add coolant: to the correct level, which is typically the "cold" fill line, being careful not to overfill. 
5. Secure the cap: tightly after adding the coolant. 

What to do if you have a low coolant warning

• If the low coolant warning light comes on while driving, pull over to a safe location and shut the engine off. 
• Let the engine cool down completely before attempting to add coolant. 
• If you need to add coolant immediately and cannot wait for the engine to cool, call for roadside assistance or a tow truck to take your vehicle to a repair shop. 

Does my car need to be running when adding coolant?

No, the engine should be off and cool when adding coolant, as a running engine creates a pressurized, hot system that can cause serious burns. Always wait until the engine has completely cooled down before opening the cap to add coolant. 
This video demonstrates the entire process of adding coolant, including the initial check and adding coolant after starting the engine: 1m2CarProsYouTube · Oct 6, 2016
Safety precautions

• Turn the engine off: Never open the system while the engine is running. The system is under high pressure when hot, and opening it can cause scalding steam or coolant to spray out. 
• Wait for the engine to cool: Allow the engine to cool down completely. This may take several hours depending on the ambient temperature and how hot the engine was when you turned it off. 
• Locate the reservoir: Find the coolant reservoir tank, which is usually a translucent plastic container with "cold" and "hot" fill lines marked on the side. 
• Add coolant to the correct line: Add coolant until it reaches the "cold" fill line. 

This video shows how to check and add coolant, focusing on the location of the coolant reservoir: 59sBackyard ChevyYouTube · Nov 27, 2021
Why you should wait for the engine to cool

• Prevent injury: Opening a hot, pressurized system can cause severe burns. 
• Avoid engine damage: Adding cold liquid to a hot engine block can cause it to crack. 
• Accurate reading: A hot engine's coolant will expand, leading to a falsely high reading. Checking and adding coolant when cold is the most accurate way to ensure the correct level. 

Can I drive immediately after adding coolant?

You can usually drive immediately after adding coolant, but it is crucial to wait for a hot engine to cool down first to avoid severe burns and engine damage. After adding coolant to a cold engine, it's best to drive for a short time to help circulate the fluid and remove air pockets before re-checking the level. 
Safety and engine care

• Cool down the engine: Never open the radiator cap on a hot engine; wait at least 30-60 minutes for it to cool to prevent burns from pressurized, scalding coolant. 
• Fill the coolant: Add coolant slowly to a cold engine until it reaches the fill line. The level will likely drop a few times as the fluid circulates. 
• Check for leaks and air: Drive for a short distance and then re-check the coolant level and look for any leaks. Running the engine helps to move air bubbles out of the system, which is important to prevent overheating, especially if you've drained the system. 

What to do if the engine overheated

• Do not drive immediately: If the engine overheated, wait for it to cool down completely before adding any coolant, and then drive with caution. 
• Allow for a proper cool-down: Waiting to add coolant to a hot engine can cause severe damage to the engine block. 

Additional tips

• Use the right coolant: Always use the correct type of coolant for your vehicle. 
• Check the manual: Consult your car's owner's manual for specific instructions and recommendations on coolant and bleeding the system.