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Coolant Reservoir Empty but Radiator Is Full (Explained)

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  • 6 min read

Last Updated on: 7th September 2023, 12:47 am

There might be instances where you find the coolant tank with low or no coolant fluid, while the radiator remains filled with coolant.

If the car hasn’t been used recently and is at a cold state, and you find that the coolant container exhibits a low level of fluid, it implies that there’s a need to refill the coolant.

On the other hand, if the car has been in operation recently, it’s possible that the coolant was sucked into the radiator due to the pressure that builds up in the cooling system.

In this article, we will delve into the reasons why there’s a difference in the levels between the coolant reservoir and the radiator, and provide additional information that may be useful.

Why Is Coolant Reservoir Empty but Radiator Full?

The coolant tank and the radiator are two distinct elements of your car’s cooling system.

The coolant tank serves as a holding place for surplus liquid, whereas the radiator’s role is to dissipate heat from the engine and ensure a stable operating temperature.

If you observe that the coolant tank is devoid of fluid but the radiator remains filled, it’s probable that due to pressure accumulation in the cooling system, coolant is being forced from the reservoir into the radiator.

This is an intended mechanism, and its purpose is to avert the coolant from reaching its boiling point and subsequently harming the engine.

However, this could also indicate that the coolant levels are insufficient. It’s advisable to inspect the indicators on the side of the coolant container to determine the appropriate levels, and replenish the coolant if necessary.

Typically, the side of the coolant reservoir will have “min” and “max” markers.

coolant reservoir in engine bay

Driving With an Empty Coolant Reservoir

If the coolant tank is devoid of fluid, this might signify that the system lacks the necessary amount of coolant to adequately cool down the engine.

Operating the vehicle with a coolant reservoir that’s empty can result in the engine getting excessively hot, which in turn can cause severe damage or even total engine breakdown.

It is imperative to replenish the coolant container promptly.

If you observe any indications of the engine overheating, like the ones enumerated below, it’s crucial to immediately find a safe spot to halt the vehicle and switch off the engine to avoid additional damage to it.

  • Elevated reading on the coolant temperature gauge
  • Vapour and steam emanating from beneath the hood/bonnet
  • Coolant temperature alert displayed on the dashboard
signs of a car overheating

Should the Radiator Be Completely Full of Coolant?

The radiator must have coolant up to the appropriate level, usually denoted by a “max” or “min” line on the side of the radiator.

If the radiator is filled beyond its capacity, it may cause the coolant to spill over, leading to harm to other components of the cooling system.

If you realise that the coolant is excessively filled, you ought to extract the excess amount. It is also crucial to inspect the radiator for any leaks or damages.

Can the Radiator Draw Coolant from the Reservoir?

Yes, the radiator will extract coolant from the tank as required to keep the cooling system at the correct coolant concentration.

The radiator cap and thermostat modulate the flow of coolant, overseeing the pressure and temperature within the system.

Why the Coolant Reservoir Is Empty When Hot

The coolant container should not be vacant while the engine is hot and in operation, as this implies a deficiency in the coolant levels.

Nonetheless, the coolant might momentarily settle at a lower level in the reservoir right after turning the engine off.

If the coolant container is empty while the engine is hot and running, this could indicate a leak, issues with coolant circulation, or insufficient coolant levels in the vehicle.

Why the Coolant Reservoir Is Empty When Cold

The coolant tank needs to be filled to the designated level when the engine is cold, as marked by the “max” or “min” indicators on the reservoir’s side.

If it is empty when the engine is cold, it is vital to refill it and investigate any potential leaks or problems with the cooling system.

What Is the Coolant Capacity of a Radiator?

The quantity of coolant that a radiator can accommodate depends on the vehicle’s make and model, as well as the radiator’s size.

On average, a car radiator has a capacity of around 3 gallons (13 litres).

It’s recommended to refer to your vehicle’s owner’s manual to ascertain the correct coolant volume for your particular model.

Where Should to Pour Coolant – Reservoir or Radiator?

You need to add coolant to the coolant reservoir, not directly into the radiator. The coolant reservoir is essentially a holding tank for surplus coolant and is constructed for convenient access and refilling.

Follow these steps below to examine the coolant level and refill the reservoir if needed.

  1. Locate the coolant reservoir within the engine compartment.
  2. Make sure the engine is cold before you loosen the cap of the reservoir.
  3. Examine the “min” and “max” lines on the reservoir’s side. If the level is below the minimum, fill it so that it is between the min and max indicators.
  4. Secure the reservoir cap.
  5. Start the engine and let it run for around 5 minutes, then once it has cooled down, recheck the coolant level.
YouTube video

Why Does the Coolant Reservoir Keep Emptying?

If the coolant tank consistently depletes, there’s a high possibility that there’s a leak within the system.

This could stem from:

  • a compromised hose
  • a malfunctioning radiator
  • an issue with the water pump
  • a defective radiator cap

It’s crucial to get a professional mechanic to examine the cooling system and pinpoint and mend the leakage.

Coolant Reservoir Depletes without Visible Leakage

In the event that the coolant reservoir is emptying but you don’t observe any apparent leakage, it might be that the coolant is being used up due to an engine issue.

This might result from a leaking head gasket, a fissured cylinder head, or an anomaly in the engine block.

It’s imperative to seek the expertise of a mechanic to analyze the engine and identify the root cause of the problem, followed by undertaking the required repairs.


Observing that the coolant tank is vacant while the radiator remains filled can be unsettling.

Nonetheless, this could either be a part of the normal functioning or an indication of an underlying engine issue.

It’s essential to monitor the coolant levels in both the radiator and the coolant tank vigilantly, and to tackle any arising problems swiftly in order to ward off damage to the engine.

Should you spot any leakages or face issues with the cooling system, or if the coolant tank keeps depleting, it’s imperative to have your car professionally inspected.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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