What happens if you overfill coolant, and how do you fix it?

In the life of many motorists, unpleasant situations have occurred with an overfill of the coolant. This can be a critical moment for both vehicle performance, health and the environment.

Coolant is a liquid that should be added to the engine compartment to keep the engine from overheating. Exceeding the permissible operating temperature of the engine leads to breakdowns.

But what if the overflow of coolant also leads to the motor overheating, its complete breakdown, and worse things?

Let’s see what happens if you overfill coolant, what to do about it, and how to add coolant correctly to prevent this from happening in the future.

overfill coolant

Where is coolant stored?

Let’s remember which way the coolant goes in your car in order to assume possible breakdowns when the coolant is overfilled.

Today, cars are equipped with two types of coolant storage tanks: expansion tank and overflow tank, which is coolant recovery.

Similar containers are located near the motor.

Let’s take a closer look at each of them.

  • An expansion tank.Refers to a closed vehicle cooling system and is under high system pressure created by the engine cooling system. This coolant reservoir is connected to a radiator.The expansion tank has a metal pressure-rated cap similar to the one on the radiator in an old car.Warning: Never remove this cap while the engine is hot! While the engine is hot and the cooling system is running, increased pressure is recorded in the antifreeze tank. Removing the cap is fraught with an uncontrolled splashing of coolant.Typically, the expansion tank requires more plumbing than the overflow tank, but this coolant reservoir is more efficient for relieving high pressures from heated coolant since it’s always being fed into the coolant tank to expand.
  • Overflow tank.The overflow tank is a coolant reservoir for normal pressure coolant. The overflow tank also connects to the radiator and has a lid. True, this cover looks like a ventilation hole.This reservoir saves coolant consumption – hot coolant cools down there and already cold coolant is drawn back into the radiator.Surprisingly, such a function appeared relatively recently – a few decades ago, the coolant simply flowed out of the filler neck drain port directly onto the road.After several such cycles, free air space forms in the upper part of the radiator, which only contributes to the expansion of the cooling.In some modern cars, the filler provision of any type of coolant tank will be provided with an overflow hose to drain the coolant down under the car by accident.

How does hot coolant expand?

When you start the engine, it starts to heat up – while the coolant also heats up and expands in volume.

When the coolant can no longer fit in the radiator due to too much volume, it flows into the coolant reservoir to cool. The cooled coolant is drawn back into the radiator.

What happens if you overfill your coolant reservoir – in detail

So, you’ve run into an overfill coolant problem, or you’ve noticed that coolant levels are too high in your cooling system and are wondering if this can be ignored.

Let’s take a look, at what happens if you have an overfilled tank.

  1. Critical pressure.

Your cooling system is not rubber and has its own load limit. If too much coolant starts to expand when heated, the pressure will build up to critical pressure and crush all thin metal parts and seams.

In this case, the coolant will pour out through the vent cap of the radiator.

In addition to this, you will become the owner of a car with a warped cylinder head, blown head gasket, and also a cracked engine block.

  1. What happens if you overfill coolant? Engine overheating.

As a result of the destruction that occurred due to increased pressure, the entire cooling system becomes unusable and the engine overheats.

To somehow reduce the temperature, the cooling fans turn on and give a slight delay in time. If they are not there, serious breakdowns occur immediately, and you may need to replace the engine.

  1. Entry of engine oil into the cooling system.

You can classify this as a separate issue if you ask, what happens if you overfill coolant.

Due to the fact that gaskets and seals become unusable, engine oil enters the cooling system and mixes with coolant. As a result, you lose oil pressure.

  1. Engine wiring is damaged: too much coolant.

If you didn’t play it safe by insulating the engine wires with a hose, or the manufacturer didn’t, the wiring will have a hard time. The increased pressure and hot coolant leaking through the engine bay can cause electrical damage or wear out wiring components.

  1. Hazard to health and the environment: what happens if you overfill coolant?

Today there is a wide variety of coolants, but they can be classified into 3 groups: inorganic acid technology, organic acid technology, and a hybrid of both technologies.

The inorganic acid technology coolant contains ethylene glycol, which is sweet in taste and highly toxic. The sweet taste attracts children and animals and leads to serious poisoning and death.

Propylene glycol dominates organic acid technology products, but it is no less toxic than ethylene glycol.

The hybrid type contains both substances, which are good for the coolant system, but terrible for the environment.

That is why it is so important to control the level of coolant in the antifreeze tank and not allow coolant to overflow.

Can the coolant be replaced with water?

If coolant harms the environment, why not replace it with water? The answer is simple – water corrodes the engine and is not recommended.

The coolant contains anti-corrosion components that contribute to the durability of the engine.

If you still had to use water under certain circumstances, flush the coolant system and fill in the coolant as soon as possible.

How to properly fill the tank with coolant to avoid overfilling coolant later

The best way to fix a problem is to avoid an overfilled coolant reservoir. Let’s see what can be done to prevent coolant overflow.

Each of the coolant tanks described above is made of translucent plastic, so the level of coolant you fill is very easy to track.

This tank will also have 2 labels that show the maximum allowable coolant level with a hot and cold engine, respectively. Usually, “Cold” and “Hot” are written next to these notches.

When filling the tank with engine coolant, there are 2 important rules to avoid overfilling coolant tank that should not be violated if you do not want breakdowns:

1. Fill the tank only when the engine is cold.

This will help avoid splashing hot coolant, which is fraught with harm to the environment and burns and messes in the engine compartment.

2. Pour coolant up to the inscription “Cold” and not higher.

The labels on the tank may be different, but you will definitely figure out how to avoid overfilled coolant tank by knowing the main principle: leave enough space on top so that the expanding coolant does not create too much pressure and overflow.

Why do I need the inscription “Hot” then? To control the engine coolant level when the engine is hot. After all, it is not necessary to open the lid to see that the coolant level is above the mark. If so, the vehicle’s cooling system is full.

How do you remove excess coolant from the tank?

What to do if the coolant has not yet spilled, but its level in the tank is above the permissible level? First, wait until the engine cools down.

While the engine is cooling – don’t waste time, check the hardware store or auto repair shop nearby.

Your task is to find in an auto repair shop a manual plastic suction pump. These small suction pumps do an excellent job of sucking excess coolant out of the tank.

Where to put the drained coolant?

If you asked yourself this question, then you are a responsible person who cares about our planet.

Store the drained coolant in a heavy plastic container with a lid. Sign it and put it in an impassable place where children will definitely not find it.

If you decide to dispose of coolant, find the right way to do it in accordance with environmental laws.


We hope this article has helped you prevent overflowing coolant, understand the consequences of such an accident, and discourage you from driving if you have too much coolant in the coolant reservoir.

Check the coolant level in the reservoir tank regularly, but never open the cap or add coolant when the engine is hot.

Be responsible when disposing of coolant – this liquid is very harmful to the environment.


What happens if you spill coolant all over the ground?

What happens if you overfill coolant into the ground? The coolant can poison ground and surface water, sources of drinking water, and lead to the poisoning and death of people and animals.

Coolant can also be contaminated with other automotive fluids and aggravate the environment with fuel or lead.

What happens if the coolant tank is full?

Either the excess coolant will spill under the car through the overflow hose, or the coolant that has leaked into the engine compartment will damage the car in the future.

What happens to coolant when the radiator cools down?

When the radiator heats up, the coolant expansion flows into the reserve tank. As the coolant cools, it shrinks and is drawn back into the radiator.

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