What Can Cause Engine Oil in the Coolant Reservoir?

In this article we will be looking at what happens when some of these automotive fluids mix in the car. Specifically we will look at the problem of engine oil making its way into the coolant system. We will answer questions such as why this is bad and what can actually cause this to happen.

Why Is Mixing Automotive Fluids Bad?

All of the fluids in our cars have their own jobs and are designed to serve those specific purposes. This is most definitely the case with engine oil and coolant.

These two fluids are very different, oil being thicker and used to lubricate engine parts while coolant is thinner and helps cool the engine. You would not be able to lubricate the engine with coolant and oil would do a poor job in the coolant system.

The point is neither can do the other's job so if they mix in their respective systems it will impact the effectiveness of each fluid. Simply put the engine would not be as well lubricated and the coolant would not be cooling as it should.

What Causes Engine Oil in The Coolant Tank?

You are planning to add coolant to your car's cooling system so you open the tank and you see engine oil floating on the surface. Bad news, that’s not where it is supposed to be and you have yourself a problem that needs fixing straight away.

There are a few reasons that this could happen so in this section we will look at some of the more common causes. As soon as you determine why the oil is there you can fix the issue and get your car running properly again.

Blown Head Gasket

The rather ominously sounding blown head gasket deserves to be feared and it can be the most common cause for oil being in your coolant system as well as other places it shouldn’t be. This part can develop leaks and when it does it can be a costly and tricky repair job.

Between the head and the engine block is a rubber or metal seal which is called the head gasket. The only purpose of this gasket is to keep a tight seal between the head and engine block. This serves to prevent the combustion air pressure from firing-up and keeps the engine oil where it belongs.

If this gasket wears out or starts to leak then the oil begins to escape ending up in various places it should not be including the coolant system. This is very bad for the engine so if the issue is the head gasket this will need to be fixed quickly.

Faulty Heat Exchanger

For the most part oil and coolant systems are kept as separate as possible but both do need to circulate the engine. In modern cars you will often find oil coolers which are cooled using the coolant system. This is used by allowing the two liquids to come close in a part known as the heat exchanger.

The oil/coolant heat exchanger should be sealed and the liquids should not come into physical contact. It is possible however for a gasket to start leaking or a crack to develop. This would allow coolant to pour into the oil pan causing coolant in the oil system.

This is an easier and cheaper fix than a head gasket and is also one of the more common causes of the issue. You would probably check this first in the hopes that this was the problem.

Cracks in the Cylinder Head

This is a less likely cause of the problem but the possibility does exist that the cylinder head may develop damage. This will happen most often if the engine has been overheating which can cause cracks and warping in the cylinder head.

The result would be that the oil starts to leak out and find its way into the coolant. It is a tricky repair which may require welding. If you need to replace it due to extreme damage you would save money using used parts.

Engine Block Cracks

This is one of those worst case scenario things, it's rare but the oil may be caused by cracks in the engine block. If you run your engine with too little oil and not enough coolant the combination over time can actually lead to cracks in the engine block.

It is very hard to fix cracks in engine blocks which will possibly mean you need a whole new engine to fix this issue. This is a warning tale for those who are lax on their maintenance if your oil is old it doesn’t work as well and your engine can overheat.

If oil is in your coolant due to an engine block crack this will be a big repair and one that may cost so much that you simply sell the car for scrap and start over.

It’s Not Oil

Sometimes what looks like oil in the coolant isn’t actually engine oil. This doesn’t make things any better because if it's not engine oil then it may be transmission fluid. Like the oil system the transmission has a heat exchanger and this can have the same problems.

Most commonly it will be coolant in the transmission fluid if you have an issue with the heat exchanger but it can also happen the other way around as well. This is a rare possibility because only certain cars use coolant to cool the transmission.

Human Error

We would like to think it could never happen but anything is possible. While intending to top up the engine oil a lack of attention could lead to engine oil being poured into the coolant system. If you were not the last person to refill fluids it is possible a mistake was made.

If you are unable to find a valid reason for the oil in the coolant it is entirely possible someone made a mistake. You can try and replace the coolant discarding the old oily mixture. If you are able to drive around without more oil getting in then it was an error.

How Do You Fix Oil in Coolant Issues?

The repair you undertake will depend on what the issue is so your first step will be to locate where the leak is. You can do this by pressure testing your coolant and engine oil systems to determine which if any has a leak.

The equipment needed to diagnose and repair these types of issues is expensive and requires advanced mechanic skills. If you are not confident of your ability to fix this then do not hesitate to take the issue to a professional.


Whether you have oil in your coolant or it's the other way around you have a problem that needs to be fixed. The problem may not be too severe but sometimes it can be a big issue which will cost a lot to repair.

In rare cases there may be such damage to your engine that the only option is to scrap the car or replace the engine completely.

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