Oil In Coolant

Oil in Coolant Reservoir – Causes, Testing & Solutions

In Coolant by 13 Comments

If you are one of those people who has a bit of knowledge regarding cars and engines and how they work, you probably know that oil is one of the most important components for your car’s engine in particular.

If you aren’t one of those people, then you should know that oil is the lubricant which determines the overall life of your car’s engine. For an engine of any kind, whether it’s a truck engine or a sports car engine, the working principle is the same considering they are either gasoline or diesel engines.

You also know that oil and water never go together and thus, oil in the coolant reservoir is never a good sign. Here is an article about what could happen if you have oil in the coolant reservoir and what you can do to prevent it.

Oil in Coolant Reservoir Causes

Oil Cooler

While oil helps to lower the friction between the internal parts and offers some cooling, Coolant is a dedicated cooling agent that actually keeps the engine itself cool and its overall temperature under control and that is why we call it the engine coolant. The coolant is poured into a car’s radiator which circulates the fluid around the engine from the outside, keeping the temperature under control and helping to keep the car from overheating. Even if you aren’t a car specialist, you would still probably know where to pour in the engine oil and where to pour in your coolant.

But what if you open your coolant reservoir to either check the level or fill it up, and find that there is oil in the coolant reservoir? If you come across such a situation, your car definitely needs your attention as that is not a normal sign.

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These are the most common causes as to why you may find oil in the coolant reservoir of your car. Let’s go into a bit of detail on each of the points to understand why it happens and how it can be avoided.

1. Damaged Head Gasket

There’s a rubber seal snuck right between the head and the engine block, in your engine which is called the head gasket. Its sole purpose is to provide an airtight seal as the head is fitted onto the block, meaning they are two different parts. The gasket ensures that the air pressure for the combustion doesn’t light-up and the oil that’s in the engine doesn’t leak out. If an engine overheats and stays heated up for prolonged periods without the cooling, the head gasket blows up causing the oil to leak out into the coolant system. This is why some oil may be getting into the coolant reservoir. This may be the reason why you may get oil in the coolant reservoir.

Replacement of the head gasket is often very expensive as you have to take the whole engine apart and I recommend checking the other parts first.

2. Faulty Oil Cooler / Gaskets

On a lot of modern cars, there is an oil cooler fitted on your car, which is cooled by the coolant. Sometimes a gasket or a crack could occur inside the oil cooler and it will cause the oil and coolant to mix up. This part is often much easier to replace and cheaper than a head gasket. It’s also a very common problem so I  recommend checking the oil cooler if you have one fitted to your vehicle. This is often causing the coolant to pour into the oil pan instead of oil in the coolant system.

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2. Cracks in the cylinder heads

Heat can damage the cylinders over time, cracking them at certain places, allowing air and oil to leak out. The fix for this isn’t as simple and requires you to change the cylinders. This may cost you depending on your car and your mechanic and is definitely an expensive fix. Although this occurance is comparatively uncommon, it can let the oil in the coolant reservoir as you drive.

3. Cracks in the engine block

If you find cracks in your car’s engine block itself, unfortunately, you will probably need a new engine block. This is the case where the engine is usually lacking the proper oiling and cooling and due to the extreme and prolonged heat build-up, the engine block eventually cracks at points, allowing the residue oil in the coolant reservoir and other places. Fixing this would be the most expensive option as one would need to completely replace the engine as rebuilding the old one may not be possible in most cases.

How to test oil in coolant problems

Coolant Pressure Tester

There is an easy way to test out the oil in the coolant system. The easiest way is to put pressure on the coolant system and see if it’s pouring out into the oil pan. To test this out, you need a pressurize tool for the coolant system. You can find one here at Amazon if you do not have one: Stant 12270 30 Pound Cooling System And Pressure Cap TesterIr?t=Askamastermec 20&l=Am2&o=1&a=B0002Srgwu.

  1. Remove Coolant Cap
  2. Set 1.5 bar pressure with the pressure tool.
  3. Let it stand for 10 minutes. If it loses pressure during that time, you have a leak on your coolant system; it can both be an internal or external leak.
  4. Check for any external coolant leaks under your vehicle.
  5. If you can’t find any external leaks but are still losing pressure, check the oil pan either with the dipstick or tap out the engine oil to see if you get any coolant in the engine oil.
  6. Remove the oil cooler if you have one fitted on your vehicle and inspect the gaskets and search for cracks. Replace it if it looks old and dry.
  7. Remove the head gasket and check for any damage. If you can’t find any damage or signs of oil mixing between the oil and coolant channels, take expert help to check for cracks in the head or the engine block.
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Why is the mixing of these liquids harmful for the engine?

Coolant and engine oil are two completely different liquids and serve a different purpose which is why the two should never be mixed together. The reason behind it relies on both the liquids’ chemical properties. Oil is a thick viscous substance while coolant is water-like. The engine relies completely on oil for lubrication which coolants and water cannot offer. That should make it clear that oil and coolant cannot go together.

The purpose of all that oil in your car

An engine is built up of lots of parts of different sizes, all working together to get your car moving. Those parts include camshafts, gears of different sizes, pistons, and bearings. Some of the parts, like pistons and gears, are tightly coupled together which create a lot of heat due to the friction when working against each other. To mitigate the friction and produced heat, oil is used. Oil provides the necessary lubrication between the iron parts and also acts as a cooling agent for these parts. That pretty much states the use of oil. A lot of heat is generated due to all the moving parts and that is where coolants come in to keep the heat under control.

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13 thoughts on “ Oil in Coolant Reservoir – Causes, Testing & Solutions ”

  1. Thank you for that very informative write-up .
    I own a diesel powered Nippon Forklift which I have been using for a while. But recently after an engine overhaul, I started observing traces of oil on the radiator cap. Could this be a gasket head or oil cooler problem?

    1. Hi I have a 2009 BMW X6 35i 3.0L engine and my check engine light comes on but the car not overheating. I open the coolant cap and I see milky oil in it plus white smoke comes from the tailpipe. I also see oil on the oil filter and oil cooler system and it run down on the side of the engine block. Y the car not overheating and what’s the cause of the check engine light coming in. I drain the coolant Andean water tru it twice then refill it and the same thing the check light comes on as the engine temperature rise up to almost 1/4 way

  2. if i replace a secondhand nissan navara d40 oil cooler twice but the car is still pushing oil to the radiator,what might be the problem

    1. My Sedona from Chevy Cruze LS overheating check engine light never came on

  3. Small amount of oil in the water reservoir but the car does not overheat

  4. Hi I have a problem with my Jetta 5 FSI 2006 it sometimes smoke for about 1 kilometer then stops an uses much oil and I noticed brownish on my coolant reservoir what could be the problem

  5. The generator did overheating and I replaced the head gasket but after two months I started seeing oils in radiator. What is the problem. Thanks

  6. Hey Bro If i’ve got Oil in my Coolant tank, But Should There Also be Water in the Oil System if its the Head Gasket ?

    1. Im driving a Mercedes Benz C200 2013 & I recently discovered oil in the coolant tank. What is the problem.

  7. I removed oil cooler and found a mixture of oil and coolant underneath. After removing the mixture I drained the oil pan and was surprised “not” to find any coolant in it!
    My question is: why not?

  8. I have a Chevrolet captiva 2.2d Ltz. The coolant surge tank liquid has gone thick and oily. There is no sign of oil in the engine oil. What are the likely causes.

  9. Hello.,
    Thank you for your page and sharing, please cc Chevrolet Spark Year 2006 – The coolant reservoir has incoming oil . But the engine oil is fluid and clean , no smoke even the spark plugs are clean, I stop driving is medium for now on , am TOLD Head gasket or total engine revision, Thanks for your advise and comments ,

  10. I have a Hyundai 130 after Ten minutes oil is pumping through my expansion bottle, and i mean pumping!
    It drives well do you think it could be the intercooler and not the head gasket ?

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