Causes and cleaning of oil mixed with coolant in cooling system

Step by step solution to fix cooling system that is contaminated with either engine oil or transmission oil.

A very frustrating situation can arise when 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.


Three possible scenarios when coolant mixes with oil

1. Oil in the cooling system creating sludge in the cooling system.

2. Coolant in the engine or coolant in the transmission, contaminating the engine oil or transmission oil and turning it into a sludge

3. Water and oil mixed in both cooling system and inside the engine or transmission.

The guide in this article is meant to solve the first scenario. The second and third scenarios might involve opening the engine or transmission up to clean it.

How to know which among engine or transmission oil has mixed with coolant

Knowing the type of oil that has entered into the cooling system will help to narrow down on the root cause of the problem. For instance, one should not begin to replace transmission oil cooler if it is engine oil that is entering the cooling system.

What causes oil to mix with coolant?

1. Leaking cylinder head gasket: This can happen when the engine overheats causing damage to the cylinder head gasket.

2. Cracked cylinder head: A cracked cylinder head can cause oil to mix with coolant at the spot of the crack. A cracked cylinder head might not be easy to diagnose easily and it may require a technician with good experience to diagnose it.

3. Faulty engine oil cooler or heat exchanger: The oil cooler has separate chambers for oil and coolant. The chambers are separated by fins that make heat transfer from oil to coolant possible. This element that separate the oil from coolant can rupture, allowing them to mix together. A common cause for this is the use of water instead of coolant solution which is meant to prevent corrosion in cooling system component.

One common practice that kills the oil cooler fast is the use of water instead of coolant in the cooling system. See coolant versus water in cooling system to see the effect of using water.

Clean coolant mixed with oil

The mayonnaise looking substance in the cooling system will look like mud with a brownish coloration and will also have ATF smell if it is oil from transmission that is mixing.

The substance will look darker and may have whitish mayonnaise in addition if it is engine oil that is mixing with coolant. For engine that is internally very clean and has fresh oil, the color will be brownish but will have the white mayonnaise too.


4. Bad oil cooler seal or O-ring: oil coolers are mated to the engine or transmission using either seal or O-ring. If the seal or O-ring is damaged or not well fitted, it can cause oil and coolant to mix together.

Below is a photo of a new seal fitted in after removing a bad seal. 

Coolant oil seal remove oil from cooling system

5. Human error (pouring engine oil into the coolant reservoir or expansion tank): This can happen as a result of ignorance, negligence or mistake.


6. A faulty radiator (for cars that their oil coolers are integral part of the radiator) can cause coolant and oil to mix in the radiator. Some cars have engine or transmission oil hoses connected to the radiator for cooling.

Have more comprehensive reading on causes of when idling or at a stop and coolant bubbling in expansion tank because overheating can lead to burnt head gasket and cylinder head crack.

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. Fixes include either cylinder head replacement, transmission oil cooler replacement, engine oil cooler replacement, head gasket replacement, oil cooler gasket replacement or radiator replacement (for cars that have radiator and transmission oil cooler as a single unit).

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



Follow these steps in order to fix a oil infested cooling system:

1. It is advisable to remove the cooling system Thermostat (if convinient) before the cleaning process begins. This step is not compulsory.

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

3. 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.

4. 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 (not everything will come out though).

5. Refill the cooling system with water, bleed then add a quarter liters of dishwasher liquid 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.

Oil cleaning dishwashing liquid

6. 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. If there is a large amount of oil in the cooling system, you can keep adding the dishwasher detergent as you flush the cooling system.

7. 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 following the correct bleeding procedure of the particular car.

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 or leave it there. Foaming solution will cause air pockets which is not good for the bearing of some type of water pumps. Air pockets will reduce the lubrication at the bearing.

There are different procedures that works 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 replaced in order to permanently fix the fault.

When you are sure that the cooling system is clean, drain all the water and fill with recommended coolant solution.

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