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Have you noticed that the coolant in your Mercedes-Benz has become light-brown as if someone replaced it with a cappuccino or something similar? If that's the case, you almost certainly have a real problem on your hands that requires immediate attention. The most common problem is that Engine Oil is Mixed with Coolant.

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This awkward-looking sludge is a mixture of oil and coolant, meaning these two liquids are coming in contact somewhere within your engine or cooling system. Still, identifying the actual culprit may not be that simple and can sometimes result in unnecessary repairs and expenses.

To help you avoid such situations, we will go to several most common faults that cause this to happen. In addition, we will find out how to determine which of them is the most likely cause and what to do to solve the issue.

Common Causes

1. Oil Cooler

Mercedes Coolant Mixing with oil white milky in reservoir

Most Mercedes-Benz engines have an oil cooler, although this is not something all car owners know about. These are radiators through which a coolant circulates and keeps the oil at an optimum temperature.

Replacing the oil cooler gasket is a simple repair that can be completed at home and will in most cases fix the problem. 

In some cases, the metal core of a cooler may crack, allowing the oil and coolant to mix together when the engine is running. And among all possible causes that would cause such symptoms, a leaking oil cooler is the most likely one. On most Mercedes-Benz models, replacing this component is a relatively easy job.

2. Blow Head Gasket

blown head gasket

Despite their differences in design and configuration, all engines have a head gasket between a cylinder head and engine block. Its job is to ensure an air-tight seal between these components that is strong enough to cope with the combustion process. It also has a lot of holes that serve as channels through which oil and coolant flow.

This is where the mixing between these liquids may happen, as the head gasket can crack if the engine overheats. Like head gaskets, the cylinder head itself may crack if overheated.

This is, by far, the most expensive and complicated failure to repair, as cracked heads need to be replaced with new ones. And to make things worse, not all workshops can determine if the head is faulty, as this can only be done using specialized testing equipment. Luckily, most Mercedes-Benz engines are not prone to these failures, although it still can happen.

3. Radiator

Mercedes Coolant Antifreeze white milky color

An engine oil cooler is not the only such component, as cars with automatic transmissions have one for the gearbox. In most cases, this cooler is an integral part of a radiator, where it may develop a crack.

If that happens, the transmission fluid will mix with coolant. To fix the issue, owners will have to replace the whole radiator.

4. Milky white under the oil cap

Mercedes oily white oil cap

This is usually caused by condensation from inside the engine which is common if the vehicle is used for short trips. Wipe off the bottom of the oil cap and take longer trips which will make the moisture evaporate.

To confirm the problem is not a blown head gasket, you can perform an oil change. If the oil is white or milky, you should check for a blown head gasket. 

Frequently Asked Questions

Can I drive my car if it mixes coolant and oil?

  • Whatever may be causing your car to mix coolant and oil, the engine may seem to be running fine. This could tempt you to drive it in such a condition for some time. There are, however, several reasons why this is not a good idea and may have serious consequences.
  • For a start, the leak may be such that it allows coolant to get into your engine or transmission. It will dilute the oil inside it and affect its ability to lubricate moving components, which causes extensive mechanical wear. And even if this doesn't happen, a coolant contaminated with oil does not take away temperature as it should.
  • As a result, your car is quite likely to overheat under severe loads, such as when towing a trailer or driving uphill. There are also some long-term consequences to consider, with the deposits within the cooling system being one of them. The longer you use your car in this condition, the layer of sludge becomes thicker and harder to remove afterward.

How can I figure out what is causing coolant and oil to mix?

  • Several different issues may cause the oil and coolant to mix, and determining the actual culprit may not be a straightforward job. There are, however, a few simple tricks that can make this a bit easier, with checking the engine oil condition being the first step. If it is clear and without any traces of any sludge, it is most likely that the engine oil cooler is leaking.
  • This is because the crack inside its core is so small that it causes a leak only when the engine reaches running temperature. And at that point, the oil is under higher pressure than the coolant. A similar will happen if the transmission oil cooler cracks, although this will not increase engine oil consumption.
  • As opposed to broken oil coolers, head gasket or cylinder head faults may cause all sorts of additional issues. This can range from misfires and lack of performance to overheating and large amounts of thick white smoke coming from the exhaust. In addition, these problems are likely to cause the oil inside the engine to become white and sludgy.

What should I do to fix this issue?

  • After identifying which component is most likely causing the issue, the obvious next step is to replace it. In the case of the engine oil cooler, this is a relatively simple job that's well within reach of average do-it-yourself out of motive enthusiast.
  • Depending on the model, this will usually come down to undoing a few bolts and fitting a new cooler with corresponding gaskets. The situation is similar with the transmission oil cooler, although this usually requires the whole radiator to be replaced.
  • Repairing a cracked head gasket or cylinder head, on the other, is a much more elaborate job. This requires extensive expertise and access to a range of specialized tools and equipment. So, if you have never done something like that, it would be better to leave this repair to a professional.

What can cause coolant to mix with engine oil?

  • There are several typical cases where coolant and engine oil may come in contact. An oil cooler, whose metal matrix can break and develop a leak, is the most likely cause. It is also possible for a head gasket or cylinder head to crack if the engine overheats. Depending on the damage, this could also cause the mixing of oil and coolant.

Can a coolant mixed with oil damage the engine?

  • If the coolant leaks into the engine, it will dilute the oil inside it. This reduces its ability to lubricate moving components and causes extensive mechanical wear. And even if this happens another way round, allowing the oil to leak into the coolant, it can still be damaging. This is because such a mixture will not cool down the engine under high loads.


As we have explained, a light-brown sludge inside a coolant tank is a sign of oil mixing with coolant.

In most gasoline Mercedes-Benz cars, this will most likely be caused by a leak within the oil cooler. Other possibilities include leaking head gaskets and cracked cylinder heads, which may happen if the engine overheats.

Whatever is causing the issue, you should fix it without further delays. Otherwise, reduced lubrication and cooling could cause additional damage to your engine and other drivetrain components.

This problem may affect any Mercedes-Benz vehicle including Sprinters equipped with the diesel engine. 




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