What causes sludge in radiators?

Updated April 17, 2017

Sludge. It's that thick, brown-coloured substance you see when you open the cap on your car's radiator. You tend to notice it more after the engine has overheated. Unfortunately, the engine overheated because of sludge build-up. If left to continue to accumulate, damage will be done, and the radiator, as well as other parts of the cooling system, will need to be replaced. One of the leading causes of breakdown is cooling system failure.


Corrosion is the most common cause of sludge build-up in a radiator. Radiators are made of metal. Over time, antifreeze degrades. As it loses its protective qualities, PH levels change and corrosion sets in. Once this begins, rust, sludge and scale build up through the entire cooling system, including the engine. The sludge can block the flow of coolant, causing the engine to overheat and leak. If left untreated, overheating and leaking can lead to very expensive repairs.

Intake Manifold Gasket

When the intake manifold gasket leaks, oil can run into the cooling system causing sludge in the coolant. In this case, the cooling system is not to blame, but if it isn't flushed properly when the intake manifold gasket is repaired, the oily sludge left behind can cause costly damage to the cooling system.


Some General Motors vehicles require a coolant called Dex-Cool. This coolant is meant to last longer than regular antifreeze, only needing to be changed every five years instead of two. Unfortunately, this new technology has caused many headaches for the owners of these cars. Dex-Cool reacts with the plastic sealing surfaces, causing the intake manifold to leak. The residue left behind by the breakdown is very sticky and thick like mud. Only a chemical flush will remove the sludge.

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