What Happens if You Overfill Coolant on a Car?

If you’ve ever had to top up your car’s coolant, you know it can be a tricky business. If you add too much, where does it go?

What happens if you overfill coolant on a car? To answer this question, we need to understand why coolant is so important and how it works in the car engine.

In this article, we’ll explore the effects of overfilling coolant, how to properly fill your car with coolant, and what to do if you’ve made a mistake. Read on for more!

What is Engine Coolant?

Engine coolant is a fluid that helps to cool the engine. It is a mixture of water and antifreeze. Antifreeze helps to stop the mixture from freezing at a very low temperature.

It also stops the mixture from boiling at very high temperatures. The Chemical composition of the engine coolant varies from manufacturer to manufacturer. These coolants help the engine to run at a proper temperature.

What are the Types of Coolant?

There are two types of coolants: Water and Anti-Freeze. Water coolant is more common and is used in cars with engine capacities of up to about 3 Litres. Water coolant is just what it sounds like. It consists of water and anti-corrosion additives that prevent water from breaking down and damaging the engine.

Anti-freeze coolant is more commonly used in larger engines, usually those with a capacity of 3 liters or more. Anti-freeze coolant resembles water coolant, but it has a few different properties. It has a much lower freezing point and doesn’t need to be replaced as often as water coolant.

How much coolant Should I Put in My Car?

We are not saying this is an exact science, but a helpful tip is to add six ounces of coolant for every one hundred miles of normal driving.

You can check the level by opening the hood and looking at the reservoir. If you add more coolant and keep an eye on the levels, your car should always be able to start on a hot day.

100 air bubbles in radiator boiling over

What Happens if You Overfill Coolant?

Coolant is a mixture of water and antifreeze. When you overfill your coolant, you increase the amount of anti-freeze in the mix, which will lower the boiling point of the coolant.

You should never overfill your coolant because doing so can cause your cooling system to boil over or get too cold or freeze.

It could be a difficult situation, depending on the vehicle and engine, and the amount of coolant overfilled. For example, if you overfill the coolant in a Toyota, the car can be driven for about a week before the engine will seize.

If you overfill an engine with a timing belt, the belt will eventually fail, resulting in massive damage to the engine. The machine will also fail if the coolant is overfilled, but the exact amount varies from engine to engine.

Overfilling the coolant has two major problems.

  • First, the coolant becomes too thin, which means the efficiency of cooling decreases.
  • Second, the coolant can leak out through the cap or out of the overflow bottle.

How to Avoid Overfilling Coolant?

Before opening the coolant reservoir cap, ensure the coolant reservoir tank is below the overflow tank in the radiator. If the coolant reservoir tank is above the overflow tank, wipe off the overflow tank and pour coolant into the overflow tank.

This will prevent overfilling and potential damage to the radiator and engine.


Overfilling the cooling system can cause the engine to run hotter than normal. The excess coolant can boil over causing it to spill from the radiator cap and overflow.

If you overfill the cooling system and the engine overheats, you may end up with a blown head gasket.