A customer recently contacted me to enquire about the flagging limits for fuel dilution used with our Castrol® Labcheck® used oil analysis program.

After a discussion with the customer to find out why he was asking, he revealed that he has a new vocational truck that is under warranty. His issue involved getting conflicting information from the dealership where he bought the truck and his Labcheck used oil analysis reports.

It turns out that this truck has a problem with diesel fuel dilution of the engine oil. Labcheck used oil sample results are consistently coming back showing elevated levels of fuel dilution, but when this customer contacted his dealership, he was told that the truck manufacturer allows up to 6% of fuel dilution, so the dealership service department refused to provide warranty coverage for repairing the issue.

After some research, I found out that this particular dealership was behind on current OEM policy, which puts a more restrictive limit on fuel dilution… up to 4% with some extenuating circumstances. If they choose, engine OEMs and customers can set different alarm limits for fuel dilution, but the Labcheck program has established limits to provide the necessary warning about potentially serious conditions that can lead to accelerated engine wear or even catastrophic failure if not addressed.

Let’s break down why fuel dilution can be a dangerous condition. Diesel fuel dilution of the engine oil occurs due to internal leakage of the fuel injection system. Causes vary with different engines, but can be related to sealing ring failure, loose connections, faulty fuel injectors, or even the result of extended periods of idling. Regardless of how the fuel enters the engine oil, the question is at what level is it sufficient to cause harm to the engine?

There are three main concerns with fuel dilution, the first being a reduction in viscosity, the second being a dilution of the additive content of the oil, and the third being the introduction of a contaminant into the oil.

Let’s start with viscosity. Among experts, viscosity is widely considered the most important characteristic of a lubricant. The way that a lubricant lifts and separates two moving bodies and keeps them from making contact, depends on the viscosity of the lubricant being correct for the speed and load placed on the moving parts inside the engine.

There is a viscosity range that will effectively work in any given engine and viscosities below or above that range can cause problems. To understand why, I need to introduce an industry term called “hydrodynamic lubrication.” This refers to the action of the lube oil in the lubrication regime between two moving bodies, which forms a “wedge” of oil that lifts the parts and keeps them separated. If the viscosity is either too low or too high, the oil will not develop this hydrodynamic wedge.

This is a fundamental aspect of lubrication that lies at the root of wear prevention. As noted above, diesel fuel introduced into the oil causes a reduction in oil viscosity. If the viscosity falls too far or if the load increases enough, then contact between the parts will occur and wear is the result. A long-term condition of fuel dilution can lead to premature failure of the engine bearings, as well as aggressive wear of high-pressure points such as the valve train and piston rings / cylinder walls. Although a main concern involves long-term damage, if the fuel dilution is severe enough, catastrophic failure can occur over a short period of time.

Wear is not strictly a result of reduced viscosity alone, as the dilution of additive content in the oil also plays a part. Engine oils are formulated with special additives that work to prevent wear. These “anti-wear” additives are surface acting, meaning they have a natural attraction to the internal surfaces of the engine. They form a sacrificial layer of protection and, in those times when the film strength of the oil itself is insufficient to keep the moving parts separated, the layer of anti-wear additives provides a sort of cushion.

Instead of “metal-on-metal” contact occurring, the contact is “additive-on-additive.” This prevents wear of the metal, but it results in part of the additive layer wearing away as it sacrifices itself to protect the engine components. When fuel dilution of the oil increases, the concentration of these anti-wear additives is also diluted and so is their ability to prevent wear.

Now, as if all of that is not bad enough, there is even more concerning news, which is the introduction of a foreign substance into the oil. The result of fuel dilution can be an increase in oxidation of the engine oil. Oxidation is a naturally occurring condition of aging that happens to all lubricants and is the main limit to the life span of lubricating oil.

As oxidation occurs, the oil darkens in color, the viscosity increases, and corrosive compounds develop that can attack the engine internally, especially the soft metals used in the engine bearings. Besides corrosion, oxidation can also result in the formation of sludge and varnish deposits.

Now we need to consider the technology that is available to help identify levels of fuel dilution in oil. Unfortunately, this is not an easy task. First of all, we’re talking about one hydrocarbon (diesel fuel) within another hydrocarbon (engine oil). Both diesel fuel and lubricating oil are predominantly made up of molecules formed of carbon atoms joined to atoms of hydrogen. The main difference between diesel fuel and engine oil is the size of the molecules or the number of carbon atoms in each.

So, once they are blended together, how do we determine how much of one exists in a homogenous mixture of the two? Different laboratories use different technologies that have differing accuracies. From a simple open-cup flash test, to Fourier Transform Infrared Spectroscopy, to Gas Chromatography, each has different strengths and weaknesses and the accuracy of each method is also different. Depending on the method used and the skill of the test operator, a single sample of used oil with a known percentage of fuel dilution can lead to a wide array of test results.

Making this even more difficult is the real problem of fuel evaporation. Diesel fuel is much more volatile than engine oil. This means that as the engine is running at operating temperature and the engine oil is exposed to the high temperatures within a working engine, some of the fuel in the oil will evaporate. So the actual fuel dilution in the oil tends to be even higher than what is measured and reported in a used oil analysis report.

Engine OEMs will let you know that they consider a certain percentage of fuel dilution to be acceptable, but do they know which test method was used in the lab and do they know how much fuel evaporation has occurred? How is it possible to assign a random number or percent of dilution without that information? What is a fleet or heavy equipment owner/operator supposed to do?

As a lubricants supplier and as a lab services provider, Castrol has two perspectives on this situation, and under both perspectives, fuel in the oil is not a good thing. Oil is not engineered with fuel as an intended component and we realize the risks that fuel dilution creates for our customers.

Because of this, fuel dilution limits have been established in Labcheck that we feel are appropriate based on real-world data. However, if custom limits need to be set, that is okay. Should you be faced with this situation, it is our hope that this article will help you to make a more-informed decision.

Please note that Castrol’s team of experienced field engineers is available to help you with fuel dilution issues or any other questions you have regarding your used oil analysis or fleet maintenance programs.