Editorial Feature

Fuel Dilution in Engine Oil - How It Happens and What It Leads To

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Fuel dilution of the engine oil is a problem that faces many modern combustion engines. Also known as “crankcase dilution” it is a process where fuel is leaked into engine oil when it travels towards the crankcase. It is a process that occurs roughly 8 times per 1000 rpm for each cylinder, and whilst it occurs in every engine, it is a factor that contributes a significant amount of wear to the engine. It is also one of the main reasons why periodic oil testing is performed for many engines, as it inevitable that every engine will experience it. In this article, we look at what the main causes of fuel dilution are and the issues that it can cause.

The Causes of Fuel Dilution in Engine Oil

There is not one cause of fuel dilution within engine oils and this is a reason why it is such a common occurrence within engines. However, the main cause is through a process called blow-by. This occurs because most seals are not perfect (nor are the cylinders perfectly rounded), or there is an opening through the cylinder crosshatches, and this causes the migration of fuel and exhaust gases into the areas where the oil is used. This often happens in the region between the piston rings and the cylinder bore.

Other factors include leaking of the fuel injectors, incomplete combustion of the fuel, low engine temperatures, long periods of idle time and frequent short-distance driving. Below, we look at some of the other factors and mechanisms that contribute to fuel dilution in engine oils.

Wet Stacking

Wet stacking occurs in engines which are cold, i.e. engines that have not yet reached their optimum operating temperature. Colder engines have a lower combustion efficiency than engines which are at the ideal operating temperature, and this causes the fuel to ignite further on in the compression stroke (because of the lower internal temperatures). This can also cause the fuel coming out of the injector to stick to the walls of the cylinder. Once the fuel is stuck to the sides, it will slowly enter the oil system by being scraped off by the piston rings and entering the crankcase.

Poor Maintenance

Fuel dilution within an oil system happens slowly. So, when some oil becomes contaminated with a small amount of fuel, the effects are very minimal and won’t affect the performance of the oil (or the engine in general). That is if it isn’t allowed to build up—that is when issues start to arise as large amounts of fuel dilution will affect the performance of the oil and the engine. Therefore, regular maintenance in the form of regular oil checks and changes can stop fuel dilution from presenting any adverse effects, but there can be catastrophic consequences if these basic checks are ignored.

As well as regular oil checks and changes, checking the fuel injector nozzles can help to reduce the occurrence of fuel dilution—as dirty nozzles can prevent the fuel from atomizing correctly, and in turn, prevent the fuel from combusting efficiently—so not keeping up with maintenance in this area is another way of increasing the potential for fuel dilution to occur.

Engine Modifications

Modifications to an engine, especially anything that affects the way that fuel is injected into an engine, is another way of increasing the possibility of fuel dilution within the engine oil. This is often manifested through modifications that produce a much greater amount of smoke. The smoke passes by the crankcase before leaving via the tailpipe, so naturally, more smoke is going to enter the oil system which causes the oil to be diluted by the fuel; as well introducing other types of oil contamination.

The Implications of Fuel Dilution in Engine Oil

There are quite a few concerns with fuel dilution which affect the performance of the oil and the engine in general. Over time, excessive dilution can lead to a significant amount of wear, and ultimately, the failure of the engine.

The biggest issue that arises from fuel dilution is the lowering of the oil’s viscosity, as fuel has a much lower viscosity than the oil (as well as possessing a lower vapor pressure and thinning effect), which in turn can cause the oil-fuel mixture to adopt a viscosity lower than it is designed for. This causes the oil to possess less-effective lubricating properties and causes the strength of the oil film to be reduced, which increases the amount of wear on the cylinder liner and the bearings—this arises from the fact that the oil film is crucial for reducing the friction between moving components by providing a barrier, and the thinning of the oil causes the effectiveness of the barrier to be reduced, thus increasing the amount of wear on the system.

There are also many other issues that can occur because of a lower oil viscosity (or a degraded oil in general), and these include reducing the effectiveness of the additives within the oil, increasing the volatility of the oil and increasing the rate at which oxidation occurs within the oil (which then leads to more frequent oil changes).


Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.

Liam Critchley

Written by

Liam Critchley

Liam Critchley is a writer and journalist who specializes in Chemistry and Nanotechnology, with a MChem in Chemistry and Nanotechnology and M.Sc. Research in Chemical Engineering.


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  1. Bill Abraham Bill Abraham Costa Rica says:

    Is there any correlation to reduced fuel efficiency when fuel dilution in engine oil is found?  For example, if you have 5% dilution fuel dilution in engine oil, would that equate to 5% lower overall fuel efficiency?

    • Michael Czajka Michael Czajka Australia says:

      Fuel dilution increases friction so the fuel efficiency would drop slightly... but not significantly. A bigger problem would be an increase in wear due to the reduced film strength.

      When we were testing we found that E10 (10% ethanol) used correlated to fuel dilution (it varied in direct proportion to how much we used). As E10 is slightly less energy dense than normal petrol it is normal to see a ~5% drop in fuel efficiency. So any drop in fuel efficiency is not from the fuel dilution but from the ethanol.

      A secondary problem is that the polar E10 causes varnish and wax from the oil to deposit around the engine. Some of these contaminants do not flush out with the oil change and can affect your performance for many kilometres.


  2. Michael Czajka Michael Czajka Australia says:

    Fuel dilution can also be increased by using E10 (ethanol 10% fuel). The polar E10 (oil is non-polar) also causes varnish and wax to deposit throughout the engine.

    The decreased film strength of the oil due to lower viscosity can be offset by using a boundary lubricant e.g. A single application boundary lubricant dropped the wear by 82% in a new engine and even with fuel dilution the wear did not change.


The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoM.com.

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