6 Reasons Why There Is Oil In Your Coolant Reservoir

Coolant should be in the cooling system, and engine oil should be in the engine. If you find oil in the coolant reservoir, you know there is a severe problem. Here's why

Coolant Reservoir Car

Were you about to fill your car with coolant when you realized that there is a lot of oil in the coolant reservoir?

You’ve probably heard before that oil together with coolant is a very bad sign, but is it actually true?

In this article, we will go through everything you have to know about oil and coolant mixed together – the common causes of it and how you can diagnose and prevent it. So let’s take a quick look at what could cause oil to be in the coolant reservoir.

6 Causes of Engine Oil In The Coolant Reservoir

The most common cause of oil in the coolant reservoir is due to a blown head gasket or a cracked oil/coolant heat exchanger. It can also be caused by a faulty transmission cooler. In rare cases, it happens because of cracks in the engine.

Before replacing any parts, you should be sure that no one filled the coolant reservoir with engine oil by mistake. Depending on the age of a car, it is also possible that a little bit of oil slipped into the coolant over the years. So if you haven’t changed the coolant in a few years, it can just be a sign that you need to change the coolant.

If you need to take a closer look, here is a more detailed list of why you may have oil in the coolant reservoir:

1. Damaged Head Gasket

Head Gasket

Unfortunately, the most common problem causing oil in your coolant reservoir is a bad or leaking head gasket. I say “unfortunately” because replacing it is often quite a large job, and is thus usually very costly.

There’s a rubber or metal seal right between the head and the engine block in your engine 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 head gasket ensures that the combustion’s air pressure doesn’t fire-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 cooling, the head gasket can blow, causing oil to leak into the coolant system. This is why some oil may be getting into the coolant reservoir.

Because of the expense of replacing the head gasket, I do recommend checking out the other parts first and making a proper diagnosis before replacement.

2. Faulty Oil/Coolant Heat Exchanger

Oil Coolant Heat Exchanger

On a lot of modern cars, there is an oil cooler installed on your car, which is cooled by the coolant. Sometimes a gasket or a crack could occur inside the oil heat exchanger, and it will cause the oil and coolant to mix.

This part is often much easier to replace than a head gasket, and cheaper as well. It’s also a very common problem, so I recommend checking the oil heat exchanger if you have one fitted to your vehicle.

This often causes the coolant to pour into the oil pan, rather than oil leaking into the coolant system.

3. Cracks in the cylinder head

Cylinder Head

Overheating damages the cylinders over time, cracking them in certain places, and allowing air and oil to leak out. The cylinder head can also be warped if the engine overheats, which will cause the head gasket to leak.

Fixing cracks in the cylinder head is often very difficult and, in a few cases, even impossible. It often requires welding, which can be difficult in some places.

It is often easier and cheaper to replace the cylinder head with another used one. Although this problem is comparatively uncommon, it can let oil into the coolant reservoir as you drive.

However, you should always check other possible causes first and really make sure that it is the cylinder head that is causing it.

4. Cracks in the engine block

Engine Block

If you find cracks in your car’s engine block itself, unfortunately, you will probably need a new engine block, as it is often very difficult to weld it.

This usually happens if the engine lacks the proper oiling and cooling. The engine block cracks due to extreme and prolonged heat build-up, allowing residue oil into the coolant reservoir and into other places, as well.

Fixing this would be the most expensive option, as one would need to replace the whole engine. Repairing or rebuilding the old engine block is often more expensive than buying a new one.

Proper diagnosis should be made before replacing the engine block, of course.

5. Faulty Transmission Fluid Cooler

Transmission Oil Cooler

Are you sure it is actually engine oil that is in the cooling system? Actually, the transmission also has a heat exchanger that cools the transmission fluid by using the coolant, which could enter the cooling system if there is a crack.

Usually, when there is a crack in the transmission fluid cooler, the coolant goes into the transmission fluid. However, it is always possible that some of the transmission fluid also enters the coolant system.

However, this is not very common, and most cars do not even have this transmission heat exchanger. Instead, most automatic transmission vehicles have a transmission cooler installed at the front of the vehicle that is cooled by the airflow when driving at higher speeds.

6. Someone added oil there by mistake

Fill Coolant

Did you recently buy the car or let a questionable mechanic do any work on your vehicle before? It’s possible that someone filled the cooling system with something other than coolant.

It may sound ridiculous, but I can tell you that it happens more often than you may think.

If you were unable to find any leaks in the oil or cooling systems, this might just be the answer to your conundrum.

How To Fix Oil in the Coolant Reservoir?

Coolant Overflow Tank

The first thing you should do if you notice oil in the coolant reservoir is to pressure test the system. If you pressure-tested the system and everything seems fine, you might want to remove the coolant from the reservoir and keep driving the car to monitor if any new oil appears in the reservoir.

Determining if there is a leak in the oil or cooling system is often quite easy. The tricky part is finding where the leak is coming from.

There is an easy way to test if there is a leak in the oil or 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 do this, you need a coolant pressure tool for the coolant system with the right adapter for your coolant expansion tank.

They are quite expensive, so you might want to let a workshop do this for you, or borrow one. You can find one here at Amazon if you want one. This is an affiliate link, which means that at no cost to you, we may receive a small commission for qualifying purchases.

  1. Remove the Coolant Reservoir or Radiator Cap
  2. Apply 22 PSI or 1.5 bar pressure with a pressure tool.
  3. Let it stand for 10 minutes. If it loses pressure during that time, you have a leak in your coolant system; it may be either 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 for any coolant in the oil by looking at the dipstick, or tap out the engine oil and examine it that way.
  6. Remove the oil heat exchanger if you have one on your car model 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, enlist an expert to help check for cracks in the head or the engine block.

Can I Seal it with a Coolant Leak Repair Additive?

As you may know by now, when the oil and coolant are mixed, it can cause a lot of further engine problems that may destroy your whole engine.

Because of this, I do strongly recommend always repairing the problem properly instead of trying any additive.

Repairing the problems causing the oil in the coolant reservoir can often be quite expensive, though, so if you have an ancient car that is just going to drive for a little bit more, you might decide you want to risk leaving the problem unaddressed. It’s up to you if you want to risk the whole engine!

Remember that these sealers can cause other important parts in the cooling system to clog as well!

Why is the mixing of these liquids harmful to the engine?

Coolant and engine oil are two completely different liquids that serve different purposes, which is why the two should never be mixed together.

The reasoning behind this lies in 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.

Magnus Sellén
Written by:

Magnus is the owner and main author of Mechanicbase. He has been working as a car mechanic for over 10 years, and the majority of them specialized in advanced car diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.

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