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Coolant: What you need to pay attention to

The wrong or lack of maintenance of the cooling system can cause expensive damage. The coolant is particularly important. Here are the most important tips to prevent the engine from overheating.

  • Check coolant only when the engine is cold.
  • If the engine is overheated, stop at all costs.
  • In an emergency, only top up with water.

The internal combustion engine is naturally hot. To prevent it from overheating, the heat is conducted to the radiator. In a pump-driven circuit, around five to ten liters of coolant flow from the cylinder head and block to the radiator. The coolant is a mixture of several basic substances: Water, monoethylene glycol, and additives that act primarily as rust inhibitors. Commercially available concentrates are used, which are mixed about 1:1 with water that is as pure and low in lime as possible.

In modern vehicles, the cooling system is designed to be almost maintenance-free. However, small leaks can cause a continuous loss of coolant that is not indicated by the sensors. This can be caused, for example, by marten bites on hoses. If the hoses are tight and the coolant level still drops, the water pump or the cylinder head gasket may be the cause. Drivers should get to the bottom of this.

Check coolant only when the engine is cold

The coolant should only be checked and topped up when the engine is cold or not warm. This is because when the engine is warm, the system is under pressure, and opening the cap can result in serious injury or scalding. If a cold coolant is added to a hot engine, cracks can form due to temperature stresses.

In the engine compartment, the coolant reservoir, which is usually white or transparent, can be identified by the warning symbol “Caution: risk of scalding”. The “Max” and “Min” markings for maximum and minimum can be seen on the side. If the engine is still cold, the coolant level should be between the two markings.

If the fluid level is too high, the car should go to the workshop. If it is too low, the system must be checked for leaks. Laymen can recognize them by puddles under the vehicle. If the level drops again a short time after filling, there is definitely a defect.

The right mixture

When it comes to coolant, it’s all about the right mixture. The right cocktail for the engine is prescribed by the vehicle manufacturer and should always be used only as specified. Engines need a mixture of water, antifreeze monoethylene glycol, and special additives, especially for sealing and against rust.

Additives protect against corrosion

Engines and add-on parts are made of different metals and can therefore corrode when they come into contact with water. The monoethylene glycol can also oxidize, producing very corrosive compounds – such as carboxylic acids. These processes are accelerated by the high temperatures in the cooling system.

Therefore, effective corrosion inhibitors and preservatives must be added to the coolant, such as nitrates, alkali salts of organic acids, and benzothiazole derivatives. Common coolant concentrates can contain up to seven percent of these materials. These are therefore very special mixtures whose chemical properties are often defined in vehicle manufacturers’ quality standards.

In an emergency, just top up with water

If the right mixture is not available in an emergency, do not fill in any other coolant, but initially only water. The correct mixing ratio with the specified coolant can be adjusted later. Concentrates are available for this purpose, which are to be mixed one-to-one with water that is as pure and low in lime as possible. There are also ready-made mixtures available from specialist dealers that already contain sufficient water.

How to Check a Vehicle's Coolant/Antifreeze?

Antifreeze Coolant

Think about the change

The coolant does not last forever. Its change is prescribed by the manufacturer. However, it is easily forgotten because it often falls into the second half of the vehicle’s life, when maintenance according to the manufacturer’s specifications often no longer takes place. Regular checking of the levels of engine oil, brake fluid, and windshield washer fluid should therefore always include an inspection of the coolant reservoir.

If the engine overheats: Stop at all costs

If the engine overheats, it is essential to stop at the earliest opportunity. Then open the engine hood to allow the heat to escape. Then the only thing that can be done is to wait: The engine block must first cool down until you can check the coolant level. If there is still enough in the tank, you can continue driving at low speed – but always with an eye on the warning lights and temperature display on the dashboard. To be on the safe side, you should head for a workshop immediately.

Refilling and changing the coolant

Refilling and changing the coolant

If the coolant level is lower than the “MIN” mark, it must be topped up. For this purpose, a coolant from a specialized dealer should be used. It is not advisable to refill with pure water.

Since coolant also loses its effectiveness over time, it should be completely replaced every two years:

Instructions for changing the coolant

What you need:

  • Watering can.
  • Coolant.
  • Collecting vessel.

Change the coolant

Step 1: Drain coolant.

  • Allow the engine to cool to prevent burn injuries.
  • Place the collection vessel under the drain hole.
  • Remove the drain plug.
  • Open a bleed screw.
  • Allow the coolant to drain completely from the vessel.
  • Tighten the loosened drain and vent screws again.

Step 2: Top up coolant.

  • Open the engine hood and the lid of the plastic reservoir.
  • Fill the coolant into the tank with the watering can until the “MAX” mark is reached. This usually corresponds to five to ten liters.
  • Screw the lid back onto the container.
  • Now check whether enough fluid has been added: Start the engine and let it run until the operating temperature is reached. If the coolant level does not drop below the “MIN” mark, everything is in order.

When refilling and changing the coolant, the following should be observed:

  • The coolant for the vehicle is specified by the manufacturer and can be found in the vehicle’s manual. Incorrect coolant can cause damage to the engine cooling system.
  • Antifreeze should not be mixed together as they may have different compositions and therefore different properties.
  • If coolant is added, the ratio of water and antifreeze changes. The mixing ratio can be checked with a meter.

The fluid should only be changed and topped up when the engine is cold! As the cooling system is under pressure, serious injuries may occur when opening the lid.

How to check you cars coolant level? - Video


How do I know if I need to add coolant?

1. The temperature gauge reads hotter than normal when the engine is running. 2. Antifreeze leaks and puddles beneath your vehicle (orange or green fluid). 3. A grinding noise is coming from under the hood of your car. 4. Flecks or bits of rust start to show up in the antifreeze/coolant liquid. 5. Steam or an odor similar to boiling maple syrup is coming from the engine.

Can you drive with low coolant?

It is not advisable to drive or run your car with low coolant because it can lead to damage to your engine.

Do you check the coolant level with the car on or off?

The most important thing when checking coolant is you need to let your car cool down before you open the system.

David Muench (Carnes Mechanical)

David Muench (Carnes Mechanical)

Hey, I’m David. I’ve worked in a cars store for 6 years. I write reviews and guides, helping people to choose the most suitable technicals and best product for them. I’m happy to finally share my knowledge of the industry here, on CarnesMechanical.

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