What Causes Diesel Fuel In Engine Oil?

A customer recently contacted me to inquire about our Castrol Labcheck used oil analysis program’s fuel dilution alerting limitations.

After a brief conversation with the customer to learn why he was inquiring, he revealed that he had recently purchased a new vocational vehicle that was still under warranty. His problem was that the dealership where he acquired the truck and his Labcheck used oil analysis reports gave him contradictory information.

This truck, it turns out, has an issue with diesel fuel dilution of the engine oil. The results of Labcheck used oil samples consistently show high levels of fuel dilution, but when this customer contacted his dealership, he was told that the truck manufacturer allows up to 6% fuel dilution, so the dealership service department refused to provide warranty coverage for repairing the problem.

After doing some research, I discovered that this particular dealership was in violation of current OEM rules, which places a stricter limit on fuel dilution… up to 4% in some cases. Engine OEMs and customers can set various fuel dilution alarm limits if they want, but the Labcheck program has set limitations to offer the essential warning about potentially dangerous circumstances that might lead to increased engine wear or even catastrophic failure if not addressed.

Let’s look at why fuel dilution is such a risky situation. Internal leakage of the fuel injection system causes diesel fuel dilution of the engine oil. The causes vary by engine, however they can include sealing ring failure, loose connections, defective fuel injectors, or even extended durations of idling. The concern is, regardless of how the fuel enters the engine oil, at what level is it sufficient to destroy the engine?

The first is a fall in viscosity, the second is a dilution of the oil’s additive content, and the third is the introduction of a contaminant into the oil.

First, let’s look at viscosity. The most significant property of a lubricant, according to experts, is viscosity. The viscosity of the lubricant must be appropriate for the speed and load exerted on the moving parts inside the engine in order for it to lift and separate two moving bodies and keep them from colliding.

Any specific engine has a viscosity range that will work properly, and viscosities below or over that range can cause issues. To explain why, I’ll use the term “hydrodynamic lubrication” from the industry. This is the action of lubricating oil in the lubrication regime between two moving bodies, where it generates a “wedge” of oil that elevates and separates the parts. The oil will not produce this hydrodynamic wedge if the viscosity is either too low or too high.

This is a crucial part of lubrication and the foundation of wear prevention. As previously stated, introducing diesel fuel into the oil reduces the viscosity of the oil. When the viscosity falls too low or the load is too high, contact between the parts occurs, resulting in wear. Long-term fuel dilution can cause early engine bearing failure as well as aggressive wear of high-pressure locations like the valve train and piston rings / cylinder walls. Although long-term damage is a major worry, catastrophic failure can occur in a short amount of time if the fuel dilution is severe enough.

Wear is caused by a combination of factors, including lower viscosity and dilution of additive concentration in the oil. Special chemicals are included in engine oils to help prevent wear. These “anti-wear” additives are surface active, which means they have a natural affinity for the engine’s interior surfaces. They serve as a sacrificial layer of protection, and when the oil’s film strength is insufficient to keep the moving parts separated, the layer of anti-wear additives acts as a cushion.

Instead of “metal-on-metal” contact, “additive-on-additive” contact occurs. This protects the metal from wear, but it also wears away some of the added layer as it sacrifices itself to safeguard the engine components. The concentration of these anti-wear chemicals is diluted as fuel dilution of the oil grows, as does their capacity to prevent wear.

As if that wasn’t awful enough, there’s more bad news in the form of the injection of a foreign substance into the oil. Increased oxidation of the engine oil can occur as a result of fuel dilution. Oxidation is a natural aging process that occurs in all lubricants and is the primary limiting factor in the life of lubricating oil.

As the oil oxidizes, the color darkens, the viscosity rises, and corrosive chemicals form, which can harm the engine’s internal components, particularly the delicate metals utilized in the engine bearings. Oxidation can lead to the production of sludge and varnish deposits in addition to corrosion.

Now we must consider the technology available to aid in the detection of levels of fuel dilution in oil. Regrettably, this is not a simple procedure. To begin with, we’re discussing one hydrocarbon (diesel fuel) within another hydrocarbon (oil) (engine oil). The majority of molecules in diesel fuel and lubricating oil are made up of carbon atoms bonded to hydrogen atoms. The size of the molecules, or the number of carbon atoms in each, is the fundamental difference between diesel fuel and motor oil.

So, how do we know how much of one is in a homogeneous mixture of the two once they’ve been blended together? Different laboratories employ various technologies with varying degrees of accuracy. Each method, from a basic open-cup flash test to Fourier Transform Infrared Spectroscopy to Gas Chromatography, has its own set of strengths and disadvantages, as well as its own level of accuracy. A single sample of used oil with a specified percentage of fuel dilution can yield a broad range of test results depending on the method employed and the skill of the test operator.

The true issue of gasoline evaporation makes things considerably more difficult. The volatility of diesel fuel is substantially higher than that of engine oil. This implies that as the engine warms up and the engine oil is exposed to the high temperatures of a running engine, part of the fuel in the oil will evaporate. As a result, actual fuel dilution in oil is often larger than what is measured and indicated in a used oil analysis report.

Engine manufacturers will tell you that a specific amount of gasoline dilution is allowed, but do they know which lab test method was utilized and how much fuel evaporation has occurred? Without that information, how can a random number or percent of dilution be assigned? What are the responsibilities of a fleet or heavy equipment owner/operator?

Castrol has two viewpoints on this situation: as a lubricant supplier and as a lab services provider. In both cases, fuel in the oil is a bad thing. We understand the problems that fuel dilution poses to our clients because oil is not designed with gasoline as an intended component.

As a result, we’ve set fuel dilution limitations in Labcheck that we believe are suitable based on real-world data. It is, nevertheless, acceptable to define custom restrictions. If you find yourself in this circumstance, we hope that this essay will assist you in making a more educated decision.

Please keep in mind that Castrol’s team of knowledgeable field engineers is available to assist you with fuel dilution concerns or any other queries you may have about your used oil analysis or fleet maintenance programs.

Why is there diesel fuel in my oil?

In recent years, the problem of fuel going through the crankcase appears to have subsided. In this regard, ANAC investigations on heavy vehicles show that more than 5% of the engine oil samples studied contained fuel, despite the fact that the concentration was higher in vehicles previous to 2009. This appears to show that combustion with today’s fuel injection methods has alleviated the problem.

In the case of light-duty cars, there have been multiple situations when the oil level has grown rather than decreased, resulting in several instances where the level has far beyond the maximum mark on the dipstick. Although this may appear to be good news, it is not: not only does the same dilution problem occur, but it is also more problematic, because the concentration of fuel in such scenarios is extremely high, causing quick wear and engine failure. If this is the case, you might notice a decline in pressure and/or power.

As a warning indicator, some manufacturers have inserted a new notch above the maximum threshold. Reduce the mileage interval for replacing the lubricant by half if you observe an increase in the excess level. This directive is found in the vehicle’s maintenance handbook and must be followed in order to keep the warranty valid.

Furthermore, as the lubricant is depleted (1 liter per 10,000 kilometers) and replaced by fuel, the concentration of additives declines, resulting in a reduction in the engine’s protective function.

Diesel fuel enters the crankcase as a result of post-injection during regeneration in diesel cars, allowing the fuel gases to reach the crankcase and aid the regeneration process by delivering heat. Because not all diesel fuel converts to gas, some of it enters the crankcase, causing the dreaded oil dilution.

In the case of gasoline cars, TGDI engines with turbo and direct injection of this fuel are already being commercialized, with the goal of boosting energy efficiency through better fuel savings and lower harmful emissions. Because these engines are becoming more common, there will be a higher percentage of gasoline in the oil, as well as higher temperatures and operating pressures for lubricants.

In summary, when fuel and lubricant combine, the viscosity of the oil is reduced, which means that in some places of the engine, the viscosity may be too low to generate an oil film capable of withstanding severe loads and speeds. Friction between the metal surfaces occurs, resulting in part wear.

As a result, it’s critical to use a properly formulated, high-quality engine oil that can survive fuel dilution as well as the added impacts of the biofuel component. The ACEA has conducted two tests in this area: the CEC L-104, which regulates the effects of biodiesel on the engine (piston, rings, and deposit formation); and the GFC-Lu-43 A11 (oxidation in the presence of biofuel), which regulates the presence of fuel and the effects it produces, particularly biodiesel.

The lubricant is better able to preserve its characteristics and increase its capacity to protect the engine in the case of dilution as a consequence of these testing.

Can diesel in oil damage engine?

There are a number of issues with fuel dilution that affect the oil’s performance and the engine’s overall performance. Excessive dilution can cause a lot of wear on the engine over time, and eventually cause it to fail.

What causes fuel to get into oil?

Your fuel mixture is excessively rich, which is the main cause of gas flowing into the engine oil.

If your fuel mixture is excessively rich, the combustion chamber will not ignite all of the gasoline, causing it to flow down the piston rings and into the oil pan.

There could be a number of sensors malfunctioning or other issues causing the fuel mixture to become overly rich. Using an OBD2 scanner, read the trouble codes to determine if any stored trouble codes indicate where you should begin your troubleshooting.

To learn more about why your engine is running rich, see our article Why Your Engine Is Running Rich.

Why do diesel engines rise with oil?

The oil level may have risen since the last time you checked or there may be too much oil on the dipstick in some circumstances. This could be caused to condensed water (from combustion), condensed gasoline, or a coolant leak, all of which are serious issues.

Fuel-diluted motor oil can significantly lower oil viscosity and thin additive concentration (due to blow-by or leaking). Diesel gasoline has a distinct odor that may frequently be recognized immediately from the dipstick.

The oil and the engine are both harmed by free and emulsified water. If your engine has the flexible fuel vehicle (FFV) option and you’re using an alcohol-gasoline fuel blend, water condensation may be more severe for short-trip drivers. It’s crucial to note that combustion produces more water than the fuel burned in your engine. The majority of the water exits out the exhaust, but if the engine is cool, a significant amount of it may condense in the crankcase.

Putting a drop of oil from the dipstick on a hot exhaust manifold is an easy approach to detect water in used motor oil. If it crackles (sounds like bacon frying), it’s a sign of contaminated water. Be aware that there’s a chance the drop of oil will catch fire. Find out more about the crackling test.

Short-run engine starts (as many as 50) are required when vehicles are transported from the assembly factory, over the ocean, and eventually to the dealer’s lot, therefore brand new autos imported from Japan may have a high oil level. An oil and filter change may be necessary in this instance.

Whatever the source of the excessive oil level, it must be addressed as soon as possible.

Overfilling your engine with oil by accident might also cause problems. The oil will be churned as the crankshaft revolves, creating aeration and the formation of persistent foam. This can result in hot motor oil, oxidation, and oil pressure loss. Pumping spongy aerated oil is difficult. It depletes the engine’s lubrication and dehydrates crucial lubricated surfaces.

What’s the worst thing to put in diesel fuel?

Defective seals, blown head gaskets, damaged cylinder heads, corrosion damage, and cavitation all cause glycol to enter diesel engine motor oils. Glycol was discovered in 8.6% of 100,000 diesel engine samples evaluated in one investigation. In a separate investigation of 11,000 long-haul trucks, severe levels of glycol were identified in 1.5 percent of samples and mild concentrations of glycol in 16 percent. Some of the dangers linked with glycol contamination are as follows:

In diesel engine oil, just 0.4 percent coolant containing glycol is enough to coagulate soot and induce a dump-out condition, resulting in sludge, deposits, oil flow restrictions, and filter obstruction.

According to one study, glycol contamination causes 10 times the amount of wear as pure water contamination.

Precipitation occurs when glycol combines with oil additives. When oil is polluted with glycol, for example, zinc dialkyl dithiophosphate (ZDDP), an important antiwear component in motor lubricants, forms reaction products and plugs filters. This results in a reduction in antiwear and antioxidant performance.

Glycolic acid, oxalic acid, formic acid, and carbonic acid are some of the corrosive acids formed when ethylene glycol is oxidized. These acids produce a quick decline in the alkalinity (base number) of the oil, resulting in a corrosive environment that is unprotected and base oil oxidation.

The reaction of calcium sulfonate detergent additives (present in practically all motor oils) and glycol contamination produces oil balls (abrasive spherical pollutants). Crankcase bearings and other frictional surfaces in an engine have been reported to be damaged by these balls.

Glycol contamination causes an increase in oil viscosity, reducing lubrication and cooling.

How do you know when your diesel needs an oil change?

The car will be the most evident indicator that there is an issue with your oil. When there isn’t enough oil in the system, your vehicle’s oil change light will illuminate, so check the dipstick to discover what’s going on. In the worst-case scenario, the check engine light will come on. This is your car’s way of informing you that things have deteriorated to the point where the engine is in danger of being damaged due to faulty parts or a lack of lubrication.

Do you check diesel oil hot or cold?

1. Make sure you park your automobile on level ground to get an accurate reading. Turn off the engine and let it cool for 10 to 15 minutes.

Manufacturers used to advise checking your oil when the engine was cold to allow the oil to settle in the oil pan. Many manufacturers have since recalibrated their dipsticks to read accurately even when the engine is warm, according to CarTalk. For vehicle-specific instructions, consult your owner’s manual.

2. Remove the hood and look for the oil dipstick with the motor turned off. It usually has a red or orange plastic tab attached to it. If a short inspection of the engine fails to yield results, consult your owner’s manual. Some contemporary autos use electronic oil monitoring and do not allow manual oil checks.