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Coolant Reservoir Empty but Radiator Full (Explained)

Updated: Feb 1

✔ This article has been fact checked.


When it comes to maintaining your vehicle's cooling system, it's important to pay attention to the levels of coolant. However, sometimes, you may notice that the coolant reservoir may be empty but the radiator is still full with coolant.


This can be a confusing and concerning situation, and it's important to understand the cause and what actions you should take.


If the vehicle is cold and the coolant reservoir has low fluid levels, it means that the coolant needs to be topped up. If the vehicle has been recently run, the coolant may have been drawn into the radiator due to pressure within the cooling system.


In this article, we will explain why the coolant reservoir is empty but the radiator is full of coolant and other useful information you should know.


Table of Contents:


coolant reservoir in engine bay

Why Is the Coolant Reservoir Empty but the Radiator Full?

The coolant reservoir and the radiator are two separate components of your vehicle's cooling system.


The coolant reservoir acts as a storage tank for excess fluid, while the radiator is responsible for transferring heat away from the engine and maintaining a consistent operating temperature.


If you notice that the coolant reservoir is empty but the radiator is still full, it's likely that the coolant is being pushed out of the reservoir and into the radiator due to the pressure build-up in the cooling system.


This is a normal process, and it's designed to prevent the coolant from boiling over and causing damage to the engine.


However, it could also be a sign that the coolant levels are low. You should check the markings on the side of the coolant tank and see where they should be and top up the coolant if required.


Usually you'll see "min" and "max" indications on the side of coolant reservoir.


Can I Drive With an Empty Coolant Reservoir?

If the coolant reservoir is empty, it could mean that there is not enough coolant in the system to properly cool the engine. Driving with an empty coolant reservoir can cause the engine to overheat, leading to serious damage or even engine failure.


It's important to refill the coolant reservoir as soon as possible, and if you notice any signs of overheating, such as those listed below, pull over and turn off the engine as soon as it's safe to do so to prevent further engine damage.


  • High coolant temperature gauge reading

  • Steam coming from under the bonnet

  • Coolant temperature warning on the dashboard


signs of a car overheating

Should the Radiator Be Completely Full of Coolant?

The radiator should be filled to the proper level with coolant, which is typically indicated by a "max" or "min" mark on the side of the radiator.


Overfilling the radiator can lead to coolant overflowing and causing damage to other parts of the cooling system. If you notice there is too much coolant then you should remove the overfilled coolant.


It's also important to check the radiator for any leaks or damage.


Will the Radiator Pull Coolant From the Reservoir?

The radiator will pull coolant from the reservoir as needed to maintain the proper coolant levels in the system. The coolant flow is regulated by the radiator cap and thermostat, which control the pressure and temperature in the system.


Should the Coolant Reservoir Be Empty When Hot?

The coolant reservoir should not be empty when hot and running, as this indicates a lack of coolant in the system. However, the coolant may temporarily be lower in the reservoir once the engine has been shutoff.


If the coolant reservoir is empty when hot and running, it's likely that there is a leak in the system, the coolant is not being properly circulated, or your vehicle has low coolant levels.


Should the Coolant Reservoir Be Empty When Cold?

The coolant reservoir should be filled to the proper level when cold, as indicated by the "max" or "min" mark on the side of the reservoir.


If the coolant reservoir is empty when cold, it's important to refill it and check for any leaks or issues with the cooling system.


How Much Coolant Does a Radiator Hold?

The amount of coolant that a radiator holds can vary depending on the make and model of your vehicle, as well as the size of the radiator.


The average amount of coolant a radiator can hold is about 3 gallons (13 litres).

It's important to consult your vehicle's owner's manual to determine the proper coolant capacity for your specific vehicle.


Do I Add Coolant to the Reservoir or Radiator?

You should add coolant to the coolant reservoir, not the radiator. The coolant reservoir acts as a storage tank for excess coolant, and it's designed to be easily accessible for refilling.


Follow the steps below to check the level and add coolant to the reservoir.


  1. Locate the coolant reservoir in the engine bay.

  2. Ensure the engine is cold and unscrew the reservoir cap.

  3. Check the "min" and "max" markings on the side of the reservoir. If the coolant level is below the minimum, top it up so it's between the min and max level.

  4. Tighten the reservoir cap back on.

  5. Run the engine for 5 minutes and check the coolant level again when its cooled down.



Why Does the Coolant Reservoir Keep Emptying?

If the coolant reservoir keeps emptying, it's likely that there is a leak in the system. This could be caused by a damaged hose, a faulty radiator, a problem with the water pump, or even a faulty radiator cap.


It's important to have the cooling system inspected by a mechanic to identify and repair the source of the leak.


Coolant Reservoir Empty but Not Leaking

If you notice that the coolant reservoir is empty but there is no visible leak, it's possible that the coolant is being consumed by a problem with the engine. This could be caused by a head gasket leak, a cracked cylinder head, or a problem with the engine block.


It's important to have the engine inspected by a mechanic to determine the cause of the issue and make the necessary repairs.


Conclusion

It can be concerning to notice that the coolant reservoir is empty but the radiator is full. However, this could be a normal process or it could be a sign of an issue with the engine.


It's important to pay attention to the levels of coolant in both the radiator and the coolant reservoir and address any issues promptly to prevent damage to the engine.


If you notice any leaks or issues with the cooling system, or if the coolant reservoir keeps emptying, it's important to have your vehicle inspected.



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