Diesel fuel in oil. Causes and solutions

There are cases, especially in diesel engines where fuel can mix with oil. So if you notice a higher volume of oil and its shade is more transparent on your dipstick, then that can mean that diesel fuel is in the oil. The bad part of this problem is that diagnosis and repair can take a lot of time and effort.

Why is diesel fuel in oil?

To answer that question, we need to know what are the main cause that of this issue.

Failing injector nozzle

Often failing injector nozzles are the most common cause why diesel engine gets into the oil.

Over time, the nozzle seal may wear out and the tightness will be weakened. For this reason, diesel fuel will be present oil. Most often, the problem is that the density of its sealing ring disappears on the nozzle itself. If more than one nozzle is failing this means that diesel fuel will get into the oil much quicker.

The spray nozzles may partially fail from time to time. In this case, the injectors will not spray fuel, but simply pour it into the engine. Because of this, diesel fuel may not burn entirely and will get into the engine. A similar situation is observed when the nozzle opening pressure decreases.

Worn high-pressure pump parts

Often if the hardened gaskets of the high-pressure fuel pump are damaged, there is a possibility that the diesel fuel goes into the oil. Other failing parts of the high-pressure pump might cause this problem to happen such as fittings, tubes, fasteners.

Also, if bad plungers of the high-pressure pump do not deliver the required pressure, then the fuel can get into the pump itself, and there, the fuel will mix with the oil. This is true mostly for older engines. In newer engines, this is no longer present.

Sometimes the problem can be the return fittings, in the copper washers available there. They may not be properly pressed down, or they may simply leak diesel fuel.

Failing seals on the diesel fuel pump

Diesel fuel pumps will always have at least one oil seal on them, no matter what their design is, that do not allow the fuel to mix with oil. Fuel pumps can have two oil seals on some cars. If the seals or one of the seals is worn-out then that means either fuel or oil will flow out from a specifical channel.

Exhaust gas regeneration system

If the exhaust gas regeneration system is not functioning properly, diesel fuel can get into the oil. Being based on electronics, if the readings of the DPF pressure and temperature sensors, the system periodically supplies fuel, which is burned in the filter and thereby cleans it.

Here there are two possibilities. Either DPF is clogged and the regeneration system simply does not work, thus diesel fuel is constantly supplied to the filter, from where it can seep into the engine crankcase, or DPF is removed, and no software modifications were made, causing the system to continue supply excess fuel to it, which again enters the engine.

Crack in the cylinder head

A rare cause mostly present in aluminum engine blocks, diesel fuel can enter through these cracks in the engine and mix with oil. In most cases, the cracks can be caused by mechanics who tighten the nozzle simply by not using a torque wrench when installing them. Because of the excess force, micro-cracks might appear that can grow in time.

Cracks occur not only in the area where the injectors are installed but also in the channels through which the fuel is supplied. These cracks are often the result of an accident, mechanical damage, etc. Therefore, you need to check not only the head but also the rail and fuel lines.

The engine does not warm-up

Another rare case. Especially in winter, it’s a known fact that diesel engine warms much slower, and if you drive short distances or the engine does not have enough time to warm, and, also you have a failing thermostat, diesel fuel will not completely burn out, and it will condense on the cylinder walls, and from there it will already flow and mix with the oil.

As I said above, this is a rare case and two conditions must be met for this to happen, a bad thermostat and insufficient warmed engine. A failing thermostat always has enough symptoms that will hopefully make the driver check it.

How to know that you have diesel fuel in the oil. Possible signs

The easiest way to know if you have diesel fuel in oil is to check the oil level with a dipstick and its color. If the oil level is too high then it means some other fluid is mixing with oil, coolant, or fuel.

If it’s coolant then you will know this because the resulting mixture will have a whitish hue and a greasy consistency. If its diesel fuel then the resulting mixture will make to oil more transparent and you will sense the smell of diesel especially when the engine is warm.

There is another case when the engine consumes oil and the level won’t rise, however you will notice the color and smell. This means another problem that needs an additional check.

You can also check if you have diesel fuel in oil by putting the oil in warm water. If the oil is clean, without impurities, then it will blur. But if even a small fraction of fuel is in it, then you will see a “iris” shape on the oil drops

Symptoms of diesel fuel in engine oil

  • Increased fuel consumption. If you do experience this problem then it will be logical that your car to consume more fuel. However, with a very small amount of fuel leak, you can’t feel an increased consumption, only when the amount of lost fuel is higher.
  • Engine performance drops. You will see a loss of performance in many situations: when pressing the gas pedal, the car accelerates slowly, when the car is loaded it will feel like it has no power, and when you drive on a hill for example.
  • Dark steam from the excess pressure relief valve. If there is diesel fuel in the oil, then steam with a clear smell of diesel fuel comes out through it.
  • Deceased oil pressure in the system. If the oil is too liquid, and its pressure is weak, the engine will jerk. And this will lead to complete damage and replacement.

Solutions for diesel fuel in oil

The consequences for this unpleasant situation is that the oil will loose its properties and the protection against friction will decrease. Also, the oil viscosity will decrease causing other important problems like motor will overheat, leaks may appear, the necessary pressure in the system will disappear, and so on. Therefore, diesel fuel must not be allowed to enter the crankcase!

Check the injectors

They are the most common cause of why diesel is present in the oil.

You can use an air compressor to check the loss of tightness in the injector seals. Compressed air is added into the rail return channel, through which fuel is supplied. In the area of ​​the nozzles, you need to add a little bit of diesel fuel so that in the event of a leak, the air goes through it and makes bubbles. The compressed air pressure should be of 3-4 atmospheres.

Replace the O-rings. If cracks are found in places where the injectors are installed, repairs are carried out in a workshop.

If the nozzles are installed under the valve cover, the return pipes must be checked and, if necessary, pressurized before removing the nozzles to avoid unnecessary work. 

If the injectors were removed, then in any case they need to be pressurized. In this case, you need to check the spray nozzle, as well as the spray quality itself. During the removal process, you need to pay attention to the presence of diesel fuel leakage in the thread of the sprayer.

Check the diesel fuel pump

Replacing the fuel pump oil seal sometimes help solve this problem.

Check the seal of the plunger pairs in the high-pressure pump

Check for increased gaps in the plunger pairs, and the compression in the engine which you can find it in the car manual. You can also check and if necessary replace the rubber seals on the pump.

Check the cylinder head

A crack in the cylinder head or in its fuel rail is a serious problem often leading to the entire engine replacement. If the crack is small you can opt to weld it, but this does not guarantee a good result.


There are some causes for diesel fuel in oil that can be quite hard and tricky to repair. However, this can be done, if you find a good car workshop that does the job correctly. If you manage to repair this problem don’t forget to do an engine flush and add high-quality oil.

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