Why is there Sludge/Slime in my Radiator?

Cooling-system issues, like radiator sludge, account for nearly 40 percent of engine failures. Clearly, it pays to take care of your vehicle’s cooling system.

Sludge/slime are one of the common symptoms of larger problems. Left unchecked, it’ll plug the radiator, heater core or fluid passages, resulting in overheating and expensive repairs.

What causes radiator sludge/slime and what can you do about it? 

• Additives dropping out: Coolant consists of a base (typically ethylene glycol or propylene glycol) mixed with additives and water. The base is primarily responsible for providing freeze and boil-over protection. The additives guard against corrosion, cavitation and scaling. Mixing of incompatible coolants can cause the additives to “drop out” of the solution and form sludge or slime.

• Contaminated coolant: A bad head gasket or cracked cylinder head can allow oil and coolant to mix, resulting in sludge. In vehicles with automatic transmissions, the engine-cooling system also cools the transmission. A breach in the system can contaminate coolant with transmission fluid.

• Corrosion: Occurs when an imbalanced coolant chemically reacts with metallic surfaces, forming reddish deposits that can appear as sludge or slime. 

Radiator corrosion
Low-quality coolants can lead to cooling-system corrosion.

Performing fluid analysis is the only way to definitively identify what’s causing sludge/slime in your radiator. The report can identify oil, transmission fluid or other contaminants in the coolant.

Fix any mechanical defects and flush the cooling system. Refill with a high-quality antifreeze/coolant.

What kind of coolant should I use? 

Let’s start with what kind you should avoid.

You’re no doubt familiar with the conventional “green” coolants found at most retailers due to their low price. They contain inorganic salts, such as nitrites, phosphates and silicates.

Inorganic salts deplete quickly – typically in two years or less – and are on the environmental watch list. Once depleted, they are the source of common cooling-system problems, like scale deposits and sludge/slime.

Green engine coolant.
Low-cost “green” coolants are the source of several problems, such as radiator sludge/slime and scale deposits.

For these reasons, most vehicle manufacturers have moved away from inorganic salts. 

And so should you. 

Instead, use a high-quality coolant that uses organic-acid technology (known as OATs). OATS coolants are much more robust and longer-lasting. They virtually eliminate drop-out, scaling and compatibility issues inherent to inorganic salts. This type of coolant can be used in a wide variety of applications, even mixing with other coolants as a top-off. 

AMSOIL Antifreeze/Coolants

AMSOIL offers three coolants, all of which offer a unique blend of organic acids. We use di-acid technology, which means both ends of the organic acid are active. This makes them work faster and form stronger bonds for enhanced protection. 

AMSOIL Passenger Car & Light Truck Antifreeze & Coolant is formulated for all passenger cars and light trucks. It is premixed 50/50 with high-quality water.AMSOIL Passenger Car & Light Truck Antifreeze & Coolant

Buy AMSOIL Passenger Car & Light Truck Antifreeze & Coolant

AMSOIL Heavy-Duty Antifreeze & Coolant is dialed in for the unique needs of on- and off-road heavy-duty applications. It is also premixed 50/50 with high-quality water. AMSOIL Heavy-Duty Antifreeze & Coolant

Buy AMSOIL Heavy-Duty Antifreeze & Coolant

AMSOIL Low Toxicity Antifreeze and Engine Coolant is a 100-percent concentrate polyglycol formulation for situations that call for a low-toxicity product.AMSOIL Low Toxicity Antifreeze and Engine Coolant

Buy AMSOIL Low Toxicity Antifreeze and Engine Coolant


  1. Deposition of sludge in the radiator has negative influence on the performance of the cooling system. So, suitable coolant should be flushed regularly to remove the deposited sludge from the cooling system and to keep the vehicle in working order. Deformities in the cooling system could bring the vehicle to a halt in the middle of nowhere. So, the condition of different components of the cooling system including the coolant used in it should be inspected regularly and if necessary should be replaced with suitable substitutes.

  2. How often should a cooling system be drained and refilled, even if a vehicle only is used about 4000 miles per year.

    1. Hi Robert,

      Every vehicle manufacturer is different, so check the recommendations in your owner’s manual.



  3. 1 – For vehicles that require HOATs, is this antifreeze compatible?
    2 – Is this fluid closer to HOAT or OAT?
    3 – Is it ok to mix with the green stuff (IAT)?

    1. Hi Steve,

      Yes, our antifreeze & coolants can be used in vehicles that require HOATs. Our products use organic acid technology (OATs). Finally, while it’s OK to mix AMSOIL antifreeze & coolants with “green” coolants, we recommend flushing the coolant system and following your vehicle’s maintenance guidelines.



  4. Is the Amsoil Antifreeze an OATs Coolant?
    Is it compatible with the 2000 Volvo S80 2.9L Non-Turbo?
    What brand of Coolant Flush do you recommend to flush out my old coolant?

  5. I would like to thank you for sharing such a nice article about the negative effects of deposition of sludge in the radiator. Such deposition influence the performance of the cooling system in a very adverse way and even causes the overheating of the engine. So, the condition of different components of the cooling system should be inspected at a regular interval and damaged components should be replaced with suitable substitutes. Coolant flush is also essential to remove different types foreign bodies and enhance the effectiveness of the cooling system.

  6. I have an 04 Duramax that is not my daily driver and used mostly for towing. Would you consider this to be a heavy duty application or would the light truck still be the coolant of choice?

  7. Now that I think about it I don’t really know what kind of coolant I put in my car. I normally just get whatever is cheap, as long as it works for my car. Would you recommend that I flush my coolant system and put in higher quality coolant so I know what is in the engine? I definitely don’t want to cause any damage to my radiator.

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