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How to clean cooling system that is contaminated with engine oil or transmission oil


A very frustrating situation can arise where either the engine oil or automatic transmission fluid finds its way into the cooling system and mixes with the coolant solution. The oil will be present all over the engine, radiator, expansion tank, coolant reservoir and hoses.

This can happen through the following faults;

Leaking cylinder head gasket
Cracked cylinder head
Faulty engine oil cooler or heat exchanger
Faulty transmission oil cooler or heat exchanger
Human error (pouring engine oil into the coolant reservoir or expansion tank)

Removing oil from the cooling system can be a very difficult task to accomplish and it is one that requires a lot of patience and perseverance.

Luckily, commonly available dishwashers are very effective in getting rid of oil from the cooling system.

Before starting this cleaning process, ensure that the cause of the coolant and oil mixing is already fixed.

The stress involved depends on the amount of oil in the cooling system.


It is advisable to remove the cooling system Thermostat before the cleaning process begins.

Open the expansion tank cover or radiator cap and pour about a quarter liters of dishwasher Detergent. Tighten the expansion tank or radiator cap back.

Start the car, turn the heater on and to full blast and run the engine until it attains operating temperature. After the engine has attained operating temperature, still run it for 5 minutes.

Allow the engine to cool for safety. Through the coolant drain plug, located mostly at the bottom of the radiator, drain out the entire content of the cooling system.

Refill the cooling system with water, bleed then add a quarter liters of dishwasher detergent again. Cover the coolant reservoir or radiator and idle the car for 30 minutes or drive around for about 20 minutes. The heater should be put ON at this stage too. Repeat this step till almost all the oil has been dislodged.

Drain out the content of the cooling system, put a hose of running water in the radiator or expansion tank to flush out the soapy water while the car is running until the water that is coming out from the drain plug or bleed plug is clear.

Remove the water from the cooling system, return the cleaned Thermostat or put new Thermostat and replace with recommended coolant solution, then bleed the system properly.

You may also change the cooling system's hoses, as many as you can.

The Detergent is meant to be in the cooling system temporarily just for the purpose of cleaning. Never forget it there. Forming solution will cause air pockets which is not good for the bearing of some type of water pumps.

There are different procedures that work but I have used the one that I explained above and it has been very effective and without any negative effect.

Note that in some situations, the engine, transmission, Radiator, seals could be damaged already and those affected parts would have to be fixed or replaced.

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