Should the engine be running when adding coolant?

No, the engine should not be running when adding coolant to the vehicle's cooling system. The proper procedure is to add coolant with the engine off and the vehicle parked on a level surface. This ensures the coolant is added safely and effectively without any risk of injury or damage to the engine.

Reasons Why the Engine Should Be Off When Adding Coolant

• Safety: With the engine running, the cooling system is under pressure, which can cause the coolant to spray out and potentially scald the person adding it. Turning the engine off eliminates this safety risk.


• Proper Filling: Adding coolant to a running engine can lead to air pockets forming in the system, which can reduce the cooling system's efficiency. With the engine off, the coolant can be added slowly and properly fill the system.


• Avoid Damage: Pouring cold coolant into a hot, running engine can cause thermal shock and potentially damage engine components. Allowing the engine to cool first prevents this issue.

Following the proper procedure of adding coolant with the engine off is an important safety and maintenance step to ensure the cooling system functions correctly and without incident.

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Should the car be running or off when adding coolant?

You can add coolant while the engine is off or on. It does not matter. I would fill it to the max line and monitor the level. If it keeps dropping, the engine and cooling system should be checked for leaks.

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 directly to the radiator?

Yes, you can add coolant directly to the radiator, and it is often the correct procedure for vehicles that have a radiator cap. This is especially true when the cooling system is empty or the overflow reservoir is dry, as it fills the system from the lowest point and helps remove air. Always ensure the engine is cool before opening the radiator cap to prevent severe burns from hot, pressurized coolant. 
This video shows how to add coolant directly to the radiator: 48sBackyard ChevyYouTube · Nov 27, 2021
Step-by-step guide

1. Wait for the engine to cool completely: This is the most critical safety step. Never open a hot cooling system, as it is under pressure and can cause severe burns from scalding steam and fluid. 
2. Open the radiator cap: Loosen the cap slowly to release any residual pressure before fully removing it. Place a rag over the cap as you open it as an extra precaution, according to this YouTube video. 
3. Fill the radiator: Add the correct type of coolant (pre-mixed or a 50/50 mix of concentrate and distilled water) directly into the radiator until it is completely full, right up to the neck. 
4. Run the engine: Start the engine and let it run. Squeeze the upper radiator hose to "burp" the system and help push out any air bubbles. Add more coolant to the radiator if the level drops as the thermostat opens and circulates the fluid. 
5. Check the reservoir: Once the engine is at normal operating temperature, fill the overflow reservoir to the "full" or "cold" line if the level is low. 
6. Secure the caps: Once finished, replace the radiator cap tightly, and then close the reservoir cap. 

This video shows how to add coolant to the reservoir and how to identify the correct fill line: 1m2CarProsYouTube · Oct 6, 2016
Important considerations

• Check your owner's manual: Always confirm the correct type and mixture of coolant for your specific vehicle, as using the wrong type can cause damage. 
• Mix concentrate with distilled water: If you are using concentrated coolant, mix it with distilled water, not tap water, for the best results. 
• Filling the reservoir: For systems with an overflow reservoir, you will typically fill the radiator to the top first, then use the reservoir to top it off once the engine has reached operating temperature and cooled down slightly. 

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.