Oil In Coolant Reservoir? Should I be Worried?

As you were going to top up your coolant, do you suddenly realize how much oil is in your car's coolant reservoir.

It is likely that you have heard it before: oil mixed with coolant can be a bad sign. But is this true?

In this article, we will go through everything you have to know about mixed oil and coolant 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 the oil in the coolant reservoir:

A blown head gasket, cracked coolant heat exchanger or damaged oil/coolant heat transferr are the most common causes of oily fluid in your coolant reservoir. You may also experience it due to a defective transmission cooler. Rarely, cracks can occur in an engine.

It is important to make sure you don't accidentally fill the coolant reservoir with oil before replacing any components. It is possible for coolant to get a bit oily over time due to the car's age. 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.

Here's a detailed explanation of the reasons oil may be in your coolant reservoir.

Six Reasons Why Engine Oil in the Coolant Reservoir

1. Broken Head Gasket

The most frequent problem that causes oil to build up in coolant reservoirs is either a leaking or bad head gasket. It is usually a very difficult job to repair and can be quite costly.

There’s a rubber seal snuck right between the head and the engine block in your engine called the head gasket. The sole function of the head gasket is to seal the engine block when the head is attached onto it.

The gasket ensures that the combustion’s air pressure doesn’t light-up, and the oil that’s in the engine doesn’t leak out. When an engine is heated for too long, and the temperature doesn't drop, the gasket will blow up, which can cause the coolant to leak. Some oil could be getting into your coolant reservoir.

Due to the high cost of replacing the headgasket, it is important that you inspect all other components and perform a thorough diagnosis.

2. Faulty Oil/Coolant Heating Exchanger

Modern cars have an oil cooler that is used to cool the car. The oil heat exchanger can become clogged or gasket-like. This will lead to the coolant and oil separating.

This is often a much simpler and more affordable part than a headgasket. It’s also a very common problem, so I recommend checking the oil heat exchanger if you have one fitted to your vehicle.

This can often cause coolant to be poured into an oil pan rather than oil from the coolant system.

3. Cracks on the cylinder head

Overheating can cause cylinder cracks and allow oil and air to escape. Overheating can cause the head gaskets to leak by warping of the cylinder heads.

It can often be very challenging to repair cracks in the cylindrical head. In some cases it may even prove impossible. Sometimes, welding is required.

Sometimes it is easier and more affordable to swap out the head of a cylinder with a used one. Even though this is rare, you may find that the coolant reservoir can leak as you drive.

You should check all other items first and make certain that it is not the cylinder head.

4. Cracks found in the 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.

The engine's oil and coolant levels are low. This causes the block to crack at certain points.

The most costly option would be to fix this engine as it would be necessary to completely replace it.

Before replacing an engine block, it is important to properly diagnose the problem.

5. A Faulty Transmission Fluid cooler

You are sure it's not engine oil. The transmission has a heat exchanger, which cools the fluid and the coolant.

It is not common for it to happen in reverse, as it often happens with low pressure transmission coolers. You will see coolant in your transmission fluid.

However, this is rare and many cars don't even have this cooler. The transmission fluid in most automatic cars is cool by the airflow.

6. It was a mistake of someone to put oil in there

Are you a new owner of the vehicle or have you had it repaired by a doubtful mechanic? Perhaps someone has already filled your cooling system with something other than coolant.

This may seem absurd but it is true.

It is possible to solve the problem if your diagnosis fails to find any oil leakage or damage to the cooling system.

What should you do if there is oil in the Coolant Reservoir

Pressure test the cooling system immediately if oil is detected in it.

You might remove coolant from your reservoir if everything looks fine. Then, continue driving your car in order to check for any oil changes.

Sometimes it is easy to find a leak in an oil-cooling system or engine. It is the location of the leak that is tricky.

It is easy to determine if the coolant or oil system is leaky.

The easiest way is to put pressure on the coolant system and see if it’s pouring out into the oil pan. This can be done by using a coolant pressure device for coolant systems with an adapter for coolant expansion tanks.

You might consider renting one or letting a workshop handle it for you. Amazon sells them if they are not available.

1. Remove Coolant Reservoir or Radiator Cap
2. The pressure tool can be used to apply 22 psi, or 1.5 bar of pressure.
3. It should be left to rest for 10 minutes. It can lose pressure or leak through your coolant system if it does not.
4. You should inspect your car for coolant leaks.
5. If you can’t find any external leaks but are still losing pressure – check for any coolant in the oil either by looking at the dipstick or tap out the engine oil to see if you get any coolant in the engine oil.
6. Take out the oil heat exchanging unit if it is present on your car. Inspect the gaskets to find cracks. It should be replaced if the oil heat exchanger appears dry or old.
7. Take off the head gasket, and inspect for damages. 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.

Do I need to seal the leak with a coolant leak repair additive?

It can also cause engine damage, as you probably know.

It is my recommendation that the issue be fixed immediately and never to resort to any type of additive.

Repairing the problems causing the oil in the coolant reservoir can often be super-expensive, though, so if you have an ancient car that is just going to drive for a little bit more – it can be worth a chance. It’s up to you if you want to risk the whole engine!

This is the Amazon recommendation if you are looking for an additive.

These sealers could cause important cooling systems parts to clog as well!

Is it harmful to the engine to mix these liquids?

Both coolant and engine oil have completely different functions and should be treated as such.

The reason behind it relies on both the liquids’ chemical properties. Coolant, on the other hand is water-like. Oil is thick and viscous. Coolants and water are not able to provide the engine with the lubrication it needs. It should also be obvious that oil and coolant can't coexist.