6 Causes Of Oil In Coolant & How To Fix It

A sufficient level of oil and coolant is very important for the efficient operation of the vehicle engine. The engine oil and coolant prevent the engine from overheating. Your engine oil shouldn’t be mixed in the coolant. However, oil can mix in the coolant with time.

The cooling system of your vehicle should ideally contain the coolant, while the engine should contain the engine oil. However, when examining the coolant, if the liquid appears to be murky or exhibits a brownish hue, it’s probable that engine oil is mixed with the coolant. This article explains the causes of oil in coolant reservoir.  

Causes of Oil in Coolant Reservoir

If your vehicle’s engine oil is mixed with the coolant tank, it’s possible that the head gasket has blown. Other potential culprits could be a cracked cylinder head or a defective oil cooler. The issue could also be due to the unintentional introduction of oil into the system, a malfunctioning transmission cooler, or a cracked engine block. 

The following are the major causes of oil in the coolant reservoir:

1) Blown Head Gasket

The head gasket is strategically situated between the cylinder head and the engine block, acting as a barrier to prevent coolant from mixing with the oil.

In cases where the head gasket is damaged, oil may find its way into the cooling channels, eventually seeping into the coolant. This leads to the formation of a brownish, sludgy substance that may be observed in the coolant reservoir and the radiator’s upper portion.

When the head gasket is blown, it produces different symptoms, such as milky white coloration of the oil, overheating engine, loss of coolant without visible leaks, and white smoke from the exhaust.

Immediate repair of a faulty head gasket is essential, as any delay may lead to engine overheating. Prolonged neglect of this issue could culminate in severe damage to the engine.

2) Cracked Engine Block

A cracked engine block is one of the major causes of the engine oil in the coolant. The engine block is one of the most important of your vehicle. This is a part that you wouldn’t want to deal with, especially since problems with it are rare in newer cars with low mileage.

However, if a crack develops in the engine block, the only viable solution is to replace the entire engine. As a result, most cars facing this issue end up being scrapped.

3) Cracked Cylinder Head

The cylinder head is usually located above the cylinders, effectively creating a cover over the combustion chamber. However, in the overhead valve or overhead camshaft engines, this cylinder head includes fuel injectors, valves, coolant channels, spark plugs, and passages for exhaust and intake.

Straight engines contain only one cylinder head that is used by all the cylinders, whereas V engines have 2 cylinder heads, one allocated to each bank of cylinders.

Nonetheless, when the cylinder head is damaged, it may lead to oil leaking into the coolant. Unfortunately, this issue doesn’t have a simple or quick fix, frequently leading to older, high-mileage cars being sent to the scrapyard.

4) Faulty Oil Cooler

The oil cooler is an important part of a car’s engine that helps control its temperature. It is commonly found in high-performance or racing cars with advanced systems.

The main function of the oil cooler is to keep the oil at the right temperature, making sure it doesn’t get too hot or too cold. But if the cooler is not working properly, it can cause the engine oil to leak into the cooling system and mix with the coolant.

5) Bad Transmission Cooler

The latest vehicles contain a transmission cooler that utilizes transmission fluid to regulate the transmission temperature, although this feature is not found in all cars. If there’s damage, it can result in transmission fluid leaking (which is an oil) into the coolant.

Inversely, it’s also possible for coolant to mix with the transmission fluid. It is recommended to inspect closely to ascertain exactly what you’re dealing with.

6) Oil Added to Coolant Reservoir

In some cases, human error could be the cause of oil in the coolant. Although professional mechanics don’t usually encounter this issue, it can easily occur if you’re not well-versed in automotive care.

When refilling fluids, it’s crucial to ensure that you are using the correct reservoir for each fluid type. There are designated filler points for both coolant and oil. If these have been inadvertently switched, it could lead to cross-contamination of the systems.

Read More: Low Oil Pressure Symptoms and Causes

How To Fix Oil in Coolant

If you observe an oil and coolant mixture in your vehicle, you can easily fix it. If you’re feeling brave, you could consider tackling the issue on your own. However, be aware that some fixes aren’t straightforward and might require professional assistance. Let’s explore some potential solutions to this issue.

1) Perform a Pressure Test

Your initial action should be to carry out a pressure test of the coolant system, which requires a cooling system pressure tester. Most drivers don’t have this tester, but you can typically rent one from an auto parts retailer. Follow the below-given steps after buying the tester:

• Turn off the engine.
• Allow the engine to cool down.
• Properly connect the pressure tester to the radiator cap.
• Manually pump the tester until the pressure indicated on the gauge matches the value stated on the radiator cap. This is generally from 13 to 16 psi.
• Allow the car to rest for half an hour.
• Conduct a visual inspection of the cooling system for any leaks and also check the gauge for variations in pressure.
• If you observe a decrease in pressure without an evident external leak, it implies the fluid is leaking internally.

2) Locate the Coolant Leakage

If you notice an external leak, you can easily find the source and repair it. But it’s not common for a component to damage, causing both external and internal leaks simultaneously.

However, it is very hard to locate the internal leaks. In such instances, your engine will need to be partially disassembled to find the main source. However, most drivers can’t repair internal leaks in their home garage setting.

3) Repair Or Replace the Faulty Part

If the head gasket is found to be the culprit, there is a procedure for addressing this. In addition to replacing the blown gasket, you’ll need to clean the cooling system to get rid of all the oil. It may also be worthwhile to check the radiator, radiator hoses, oil pump, and water pump for damage, as the oil that has been circulating is thicker than coolant and could have caused harm.

If there is any other component that needs replacement, it would be best to do so now. Some individuals opt to install a used engine or rebuild the existing one if an engine block crack occurs, but the vehicle still has a considerable lifespan remaining.

Regardless of the nature of the repair, the process will likely be expensive. For instance, the average replacement cost of a head gasket is between $1,500 and $2,100, owing to the complexity of the procedure. If a rebuilt engine is required, the price could vary from $2,100 to $6,200, which varies according to your vehicle model.

4) Flush the Cooling System

If you find coolant in oil, it is important to flush the cooling system after fixing the issue. Follow the below-given steps to flush a cooling system:

• Park your vehicle on a level surface.
• Let the engine cool down completely.
• Elevate the front of your vehicle using a jack.
• Clean the radiator using a hose and some soapy water to prevent any contaminants from entering the system.
• Conduct a thorough inspection of the radiator.
• Position a coolant pan beneath the drainage valve to collect the old coolant.
• Open the drainage valve and allow the old or dirty coolant to discharge.
• When the flow reduces to a drip, use clean water to flush the radiator. Refill the radiator with clean water and secure the radiator cap.
• Operate the engine for about 13 minutes.
• Turn off the engine and let it cool. Now, repeat the above steps to drain the water.
• Once completed, close the drainage valve and refill the coolant reservoir with fresh coolant.

Read More: How to Flush a Radiator?

5) Contact A Professional

If you find any of these tasks challenging, it’s best to ask a professional for help. It’s recommended to have an expert check the problem instead of risking mistakes.

Since some repairs can be expensive, it’s a good idea to get another mechanic’s opinion. You don’t want to give up on your car and junk it if the issue is actually simpler than you thought.

FAQ Section

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• Bad Coolant Temperature Sensor Symptoms and Causes
• Bad Oil Pump Symptoms and Causes
• How to Fix Radiator Leaks?
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