Does the Car Need to Be Running When Adding Coolant?

If the engine temperature on your dash indicates that it is hotter than usual, you need to check your coolant. Is the coolant level still enough, or is it already lower than the minimum? If it’s below the minimum, does the car need to be running when adding coolant?

The best time to add coolant is when your engine is cold. If you’ve just driven your car, allow the engine to cool down before removing the radiator cap. The coolant is critical in maintaining the proper engine operating temperature to prevent it from overheating.

You should allow your engine to cool down for one to two hours before adding the coolant. This should be enough time for you to open the radiator cap safely. If you want to check the coolant level, you need to wait three to five hours. Since the coolant is still warm, the temperature reading will still be high because of liquid expansion.

Read on to learn more about the right time you can add coolant to your engine and the dangers of doing it incorrectly.

Does the Car Need to Be Running When Adding Coolant?

does the car need to be running when adding coolant

Shut Off the Engine and Let It Cool Down Before Adding Coolant

The right time to add coolant to the engine is when it is cold and not when it is running. You need to shut off your engine and then wait until it cools down enough for you to remove the radiator cap safely. This will release the heat that has accumulated inside.

Prevents the Engine from Overheating

Maintaining the correct coolant level prevents your engine from overheating while driving your car. It is also essential in preventing the formation of corrosion on the internal parts of your engine.

If you want to check the coolant level in the reservoir, you should wait 3 to 5 hours. You will get a misleading reading when the coolant is still hot because of liquid expansion.

Can You Drive After Adding Coolant?

If you’re wondering if you can drive right away after adding coolant, you can if you have just topped it up. However, if you want to change or replace the old coolant, you must wait until the engine gets cold and the old coolant has settled all over the cooling system.

Open the Radiator Cap or Coolant Reservoir Cap

When adding or topping up your coolant, your radiator cap or coolant reservoir cap should be fully open. Then add the coolant slowly with a mix of half antifreeze and half water. Continue filling up the reservoir until it reaches the “full” level.

Additionally, you should add coolant to the small overflow reservoir usually mounted on the radiator’s side. Don’t forget to put the radiator or reservoir cap back in its place before turning the engine on.

Is It Safe to Use Water as a Coolant?

You can use water temporarily but not as a permanent coolant. So, you can add it only if you don’t have any coolant available.

However, replace it with a genuine coolant as soon as possible. Water cannot protect your engine effectively. Coolant can because it is formulated and designed to fulfill that function.

Why Is It Not Safe to Add Coolant When the Engine is Warm?

Adding coolant when the engine is still warm is unsafe because of the risks involved. What are these risks?

1. High Pressure

The pressure circulating inside the cooling system is high while the engine runs. If you open the radiator or coolant cap at this time, that high pressure will be released suddenly.

2. Risk of Burn Due to Steam

And if you are facing the engine, the bursting steam and hot water will explode on your face and shoulders. You may get burned.

3. Mess

Even if you can avoid the steam and hot water, your engine bay and its surrounding areas, including your garage or parking space, will be a mess if this happens. You will have a lot of cleaning to do after that.

When Is It Safe to Add Coolant?

does the car need to be running when adding antifreeze

Factors to Consider Before Adding Coolant to the Engine

There are two factors to consider before safely adding coolant to your engine, including:

  • Engine temperature, and
  • Level of the coolant in the reservoir.

How Does the Car’s Cooling System Work?

Your car’s cooling system works this way: When the coolant reaches a specific temperature, the thermostat opens up. This will allow the coolant to circulate inside the radiator.

The air penetrates the hundreds of radiator fins and cools down the coolant. Then the coolant recirculates again in the cooling system.

While the coolant is circulating inside the engine, it becomes hot and expands. This pressurizes the cooling system. The thing that releases the pressure buildup is the radiator cap. It opens up and releases the pressure according to the pressure set by the cooling system.

As the radiator releases the pressure, it draws coolant from the reservoir. The radiator is always full of liquid (water plus coolant). Since the cooling system is hot and pressurized while driving your car, you must be careful when correcting the coolant level.

You could quickly get yourself burned if you are not careful. Remember that your vehicle’s coolant is part of the car components mounted on the hood. This is a place where it gets boiling while the engine is running.

Add Coolant When the Engine Is Cold

It’s different if you have just turned on the ignition and started a cold engine. In this case, you can safely open the radiator cap and add coolant to the reservoir without any risks. However, don’t do this if you have just turned off the engine after a long drive, don’t do this because the chances are great.

Wait for At Least One to Three Hours When Working with a Hot Engine

You should at least wait 1 to 3 hours before you can safely tinker with the engine and its cooling system.

Again, does the car need to be running when adding coolant? A cold engine does not require a running engine while you add coolant. If the engine is hot, then allow the engine to run to prevent thermal shock due to a cold coolant. Alternatively, you can let the engine cool down for safety.

How Long Should You Wait After Adding Coolant Before Driving?

After you have added the coolant, you can start the engine anytime you like. If the engine is already cool and the coolant in the cooling system, there’s no risk in starting your engine and driving away to your destination.

When the air temperature hovers from 25°C to 30°C, it will take one hour or more for the temperature to drop. Still, the actual time that the temperature could drop depends on several factors:

  • The degree of heat while the car is running
  • The volume of coolant inside the cooling system
  • The ambient temperature
  • The material of the engine block. Steel engine blocks cool down slower than Aluminum engine blocks.
  • The amount of heat insulation in the engine bay

What Should You Do After Adding Coolant?

does the car need to be running to add coolant

1. Install the Reservoir Cap

After adding or replacing the coolant on the reservoir, you need to install its reservoir cap and the reservoir cap correctly and securely. This is important for your car’s engine temperature gauge.

The exact engine temperature measurement can only be done if these caps are fully closed so no other outside factors can affect the current engine temperature.

2. Monitor the Engine Temperature

After adding or replacing the coolant, you should monitor the engine temperature. This is why you also need to ensure that the engine temperature gauge is in good working condition. If it works efficiently, you can rely on the temperature indications on your dash.

3. Check the Thermostat

The thermostat of the coolant or cooling system plays an integral part in getting the correct information about the engine’s temperature. There are certain things you need to check in this thermostat to determine if it is sending the correct information to the ECU of your car:

If the coolant system’s thermostat fails, it will send the wrong data to the ECU. Your temperature readings will be incorrect. Have it checked or replaced if necessary, or your engine will overheat.

If the temperature gauge indicates a cold engine temperature even after you have been driving for a couple of hours, the thermostat is stuck open. As such, the engine can overcool. This will result in a low-temperature reading. If this happens, you have to replace the thermostat.

What Are the Dangers of Driving with Low Coolant Level?

You are risking several things if you will insist on driving your vehicle even if its coolant level is beyond the minimum:

1. Risk of Overheating

Your engine is liable to overheat. If that happens, you can damage your engine. Fixing this issue will cost you a lot of money. It is much cheaper to top up or replace the coolant.

2. Head Gasket Leak

One of the first things that will be affected if your engine overheats are its head gasket. It can blow up or develop leaks because of the extreme heat it will be subjected to. This issue usually costs around $1,000 to fix.

3. Need for Engine Replacement

If the engine damage is extensive, you may have to replace it entirely. The average cost of engine replacement nowadays is about $4,000 and will depend on the type and quality of the engine that you need to replace.

Do You Have to Wait Long After Adding the Coolant Before Driving?

It depends on your situation. You can drive your vehicle immediately if you are just topping up the coolant level. However, if you are entirely changing the coolant, you must wait for a few minutes before driving away.

You have to allow the coolant to circulate all over the coolant system. That can take several minutes to an hour, depending on the age of your vehicle.

In Closing

The best and safest time to add coolant is when the engine is cold. After you have shut off the engine, allow it to cool down before removing the radiator from the cap.

Maintaining the coolant is essential in preventing the engine from overheating. It also prevents rust from forming in the internal parts of the engine.

You need to cool down the engine for one to two hours before you can add the coolant. This is the safest way to open the radiator cap to release the accumulated heat.

If you need to check the coolant level, you must wait 3 to 5 hours. The coolant will still be warm, and the temperature gauge will still be high because the liquid has expanded.