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Gasoline in oil: the reasons why it gets in and how to determine it in the engine crankcase

By admin

Gasoline in oil leads to a decrease in the viscosity of the lubricant, as well as a loss of its performance characteristics. As a result of such a problem, the engine starts to start badly “hot”, its dynamics decreases and the fuel consumption of the car as a whole increases. There are many reasons why gasoline appears in the crankcase – partial failure of the fuel pump (on carburetor engines), loss of tightness of gaskets, decrease in compression, and some others. It is possible to determine the exact reason why gasoline enters the oil even in a garage. There are several proven methods for this..


  • Signs that will allow you to find out if there is gasoline in the oil
  • Reasons why gasoline ends up in engine oil
  • How to determine
  • How to fix the problem

Gasoline in oil: the reasons why it gets in and how to determine it in the engine crankcase 1

How to understand if there is gasoline in the oil (signs)

There are ten main signs that there is gasoline in the engine oil.

  1. The oil smells like gasoline. This is usually clearly felt when checking the lubricant level in the crankcase. You can sniff both the dipstick and the filler hole. The smell is especially noticeable when the engine is warm. Often the smell is not gasoline, but acetone..
  2. The oil level rises gradually despite the fact that it was not topped up in the crankcase. This usually does not happen abruptly, but gradually, as the vehicle is used in the long term..
  3. Increased fuel consumption (gasoline) in parallel with an increase in the oil level.
  4. Oil becomes thinner. That is, it loses its viscosity. This can be determined simply by touch, by trying the composition with your fingers on a dipstick. Or just see that the oil began to drain easily from the dipstick, although this had not been observed before.
  5. Reducing oil pressure. Moreover, this fact may be accompanied by a simultaneous increase in its level in the crankcase. This is due to its liquefaction (especially important for viscous oils).
  6. Difficulty starting the engine “hot”. This is due to the loss of oil viscosity.
  7. Loss of engine power. This is reflected in a decrease in dynamic characteristics, as well as a loss of traction (the car accelerates poorly, does not pull uphill). Due to the increase in friction between the parts of the KShM.
  8. Spontaneous increase in engine speed at idle. Typical for injection engines.
  9. Errors in the ECU memory. In particular, they are associated with the formation of a rich air-fuel mixture, misfiring, as well as malfunctions of the lambda probe (oxygen sensor).
  10. Exhaust fumes acquire a harsher, fuel-like odor. Sometimes at the same time they acquire a darker shade..

Please note that the last three signs may indicate other breakdowns in the car’s engine, so it is advisable to carry out a full diagnosis, first of all, using the use of diagnostic scanners. The problem with the ingress of fuel into the oil is also found in diesel power units, however, it is determined by the same signs, but the reasons for these two types of engines will be different.

Reasons why gasoline is in oil

There are many reasons why gasoline got into the oil, including they depend on the type of engine fuel system (carburetor, injection, direct injection). Let’s consider them in order, and start with an injection gasoline engine:

Gasoline in oil: the reasons why it gets in and how to determine it in the engine crankcase 2

  • Use of low-quality fuel. It can damage the seals through which fuel will seep into the engine over time. In addition, the air-fuel mixture created from it can damage the surfaces of cylinders, pistons, valves.
  • Use of low-quality additives. Poor fuel additives can damage the gaskets. Therefore, you need to approach their use with an understanding of the matter and choose the right one or another means.
  • Worn cylinder piston rings and poor compression. This usually happens for natural reasons as a result of long-term use of the car, or due to mechanical damage. For this reason, the fuel flows directly into the crankcase where it mixes with engine oil..
  • Faulty EGR system. Improper operation of the exhaust gas recirculation system can also cause gasoline to enter the oil..
  • Skip nozzles. For engines with direct fuel injection (for example, TSI), if the injectors are missed, a small amount of gasoline from them will seep into the engine oil when the engine is started. So, after parking with the ignition on (when the pump generates a pressure of up to 130 bar), the pressure in the fuel rail helps gasoline to enter the combustion chamber, and through the gap in the rings – into the oil. A similar problem (albeit to a lesser extent) can be in ordinary injection engines..
  • Defective vacuum fuel regulator. If it does not work correctly, part of the fuel returns to the engine and mixes with oil through the gaps.
  • Rich air / fuel mixture. The formation of a rich mixture can be due to various reasons. On injection engines, this is due to a malfunction of sensors or injectors, and in carburetor machines, the carburetor may simply be incorrectly configured.
  • Defective ignition coil / spark plug / high voltage wires. The result of this is that the air-fuel mixture in a particular cylinder does not burn out. The air naturally evaporates, and the fuel vapor remains on the cylinder walls, from where it enters the crankcase.

Let’s consider separately the reasons for carburetor engines:

  • Damage to the fuel pump diaphragm. This can happen due to natural reasons (aging and wear) or as a result of mechanical damage. The lower part of the diaphragm is designed to protect its upper part from harmful crankcase gases. Accordingly, if a layer is damaged, a situation may arise when gasoline will seep into the crankcase, mixing there with lubricant.
  • Needle valve problems. Over time, it can also get damaged and work incorrectly, passing gasoline.
  • Incorrect carburetor setting. As a result, gasoline can overflow into the carburetor, including the formation of a rich air-fuel mixture. And in case of damage to the diaphragm, the situation is only aggravated.

How to identify gasoline in oil

Any car owner can determine if there is gasoline in the oil during a standard procedure in the morning before starting the engine. This can be done using one of the methods below..

Check smell

The simplest test method that will allow you to find out gasoline in oil is smell oil when checking the level with the dipstick or by unscrewing the oil filler cap. If the oil in the engine smells like gasoline, this should alert you and force you to perform several other checks. Note that oil may smell not gasoline, but acetone. It depends on the quality of the gasoline and oil used, the condition of the lubricant and other reasons..

Drip test

Gasoline in oil: the reasons why it gets in and how to determine it in the engine crankcase 3

Often, with a change in the smell of the oil, it becomes more liquid, that is, it begins to easily drain from the dipstick. You also need to pay attention to this, especially if the oil was filled for a long time, for example, the mileage on it is already more than the middle of its operational life. Therefore, in addition to the lubricant for smell, conduct a drip test to determine the quality of the oil..

So, to perform it, you just need to drip a few grams of the tested lubricant onto plain paper. You will not get an instant answer, because you need to leave it in a warm place for at least a couple of hours (preferably 12). But, having analyzed the spreading zones (along the edges of the circle there will be a sector that has a yellowish or reddish tint), then with a high degree of probability gasoline gets into the oil or not.

And in order to reduce the error of suspicion to zero, it is worth taking a closer look at the signs considered above and checking for combustion.

Burning engine oil

Many experienced motorists, in order to find out if gasoline is in oil, simply suggest setting fire to the lubricating fluid. Inexperienced drivers, who have never encountered such a problem, often mistakenly try to simply ignite the oil directly on the dipstick. This approach will not give a result, except that there is already a critical part of gasoline in the oil, but this rarely happens, and even this will be seen by other, obvious, signs.

Gasoline in oil: the reasons why it gets in and how to determine it in the engine crankcase 4

In fact it is necessary to set fire to the oil heated in a test tube. So, for this you need to take a glass test tube with a narrow neck and pour a small amount of oil into it. If the tube has a flat bottom, then it is better to warm it up on an electric stove. If the test tube has a rounded bottom, then you can take it in laboratory tongs and heat it on an open fire source (stove, candle, dry alcohol, etc.). Please note that during the heating process, the neck (upper part) of the test tube must be hermetically sealed with a cap so that gasoline does not evaporate during heating..

The ignition temperature of engine oil vapors is much higher than that of gasoline vapors, so the oils will not burn under normal conditions. Further, after a certain amount of time, when the test samples are sufficiently heated, it is necessary to open the lid of the test tube and quickly bring an open flame source (lighter, match). If the escaping vapors did not ignite, then most likely there is no gasoline in the oil or its amount is negligible. Accordingly, if the presence of gasoline is serious, a tongue of flame will appear on the neck of the test tube. In this case, it will be the result of burning gasoline vapors emanating from the lubricating fluid in the test tube..

Observe safety and fire safety regulations during the described tests.!!!

What to do when gasoline gets into oil

Gasoline in oil: the reasons why it gets in and how to determine it in the engine crankcase 5

If you find that there is fuel in the engine oil, then the first thing to think about is diagnostics to determine the cause and change the oil itself. You cannot operate the machine for a long time in this mode.!

The search for fuel leaks in the engine oil begins with checking the compression, injector seals and their performance. Diagnostics of injectors can be done with or without dismantling. On carburetor cars, it is required to check the carburetor setting, less often they replace its needle mechanism and saddle assembly.

In parallel with checking the operation of the fuel system, the system should be unscrewed and checked. The color of carbon deposits and their condition will allow you to judge the operation of the ignition system.

What are the consequences of operating a car with gasoline in oil

But what will happen if gasoline gets into the oil and cannot be detected in time? Can the car be operated under such conditions? We will answer right away – you can exploit it, but not for long.

This is due to the fact that fuel, getting into the crankcase, significantly dilutes the lubricating fluid, thereby violating its performance. A decrease in viscosity leads to poor-quality lubrication of individual parts of the motor, this is especially true when it is operating at high temperatures and at high loads. In addition, gasoline neutralizes the effect of additives in it..

A change in the composition of the oil leads to engine wear and a significant reduction in its total resource (up to overhaul).

In the most critical situations, the oil in the engine can simply ignite with all the ensuing consequences.!

Therefore, in order not to bring to the occurrence of such situations and to preserve the engine resource as much as possible, it is necessary to carry out diagnostics and appropriate repair measures as soon as possible..

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