Diesel In Oil—How It Happens and Its Consequences

Diesel in oil is a type of fuel dilution—a term used to describe the increase of oil level in the engine. This occurs when diesel fuel leaks into your engine oil when it travels towards the crankcase.

It is a normal phenomenon that occurs eight times per every 1000 rpm on each cylinder. And while this happens to every engine, it is one of the factors that causes increased engine wear and tear.

It is also one of the many reasons we perform oil testing on many engines, as it is inevitable that every car engine will experience it at one point or the other. You will learn why it happens, diesel in engine oil symptoms, and what to do in each situation.

diesel in engine oil symptoms

What is diesel fuel, and how does it work in a diesel engine?

Diesel fuel is a combustible liquid used as fuel in diesel engines. It is typically extracted from a fraction of crude oil that is less flammable than petrol.

Most heavy-duty vehicles such as boats, tractors, buses, Vans, RVs, farm applications, construction, and military vehicles use diesel fuels. Diesel generators in remote villages such as Alaska, among other villages around the globe, use diesel to generate electricity. Big hospitals, institutional facilities, large buildings, and industrial facilities use diesel to generate light with their diesel generator as backup power supply.

Most of the diesel fuel produced and consumed in the United States is refined from crude oil from petroleum refineries. According to the Environmental Impact Assessment (EIA), petroleum refineries in the state produce an average of 11 to 12 gallon of diesel from each US barrel (42 gallons) of crude oil. The United States also produces and consumes biomass-based diesel.

Typically, the spark plugs in petrol engines ignite the combustible fuel in the combustion to start and keep the engine running. But that is not the case with diesel engines. Instead, as the fuel injectors inject atomized fuel into the cylinders, the pistons compress the air, igniting the diesel fuel when the temperature becomes hot.

Diesel fuel produces more energy than the same amount of petrol fuel. This means that diesel is more economical than gasoline engines.

What are the common causes of diesel fuel getting into the oil?

Diesel in oil occurs due to internal fuel leakage from the injection system parts. Causes can vary from one engine to the other, but they are usually faulty fuel injectors, failing rubber sealing, loose connections, or as a result of long idling.

In other cases, cracks in the cylinder head, issues with the exhaust gas regeneration, driving short distances, and worn high-pressure pump parts could be the cause.

Failing fuel injector

One of the common causes of diesel mixing with engine oil is faulty fuel injectors. Over time, the injector nozzles’ seals will wear or become bristle. If this happens on a diesel engine, you will notice a diesel in oil.

The quantity of diesel that will enter the engine oil depends on how many faulty injections you have. The more faulty injections, the more the diesel in oil. The rubber seals can also be damaged due to mechanical impact when working on or installing the injectors.

Exhaust gas regeneration system

If the exhaust gas regeneration system is malfunctioning, it can cause diesel fuel to mix with the engine oil.

If the exhaust gas regeneration system is faulty, there are two possibilities. First, the diesel filter particulate (DFP) is clogged, and the system is not working. When diesel enters the DFP, it’ll remain there, and since the filter is clogged, the diesel can seep into the oil pan.

The second possibility is that the DFP was removed, and there were no modifications to fill the gap. As a result, the system will continue to supply too much fuel to it, which will find its way into the crankcase.

Engine modifications

Engine modifications are another reason diesel enters into the oil crankcase—especially any mod that affects how the injectors spray fuel into the cylinders. Also, it is common with any engine mod that produces a large amount of exhaust smoke. The excess smoke usually passes by the oil pan before exiting the engine from the exhaust tailpipe. The smoke passing through the exhaust system will dilute the oil with fuel.

Wet Stacking

Wet stacking usually occurs on engines that hardly reach their optimal operating temperature. Engines have a lower combustion efficiency when they are cold compared to when they are hot. Colder engines ignite further in the compression stroke because the temperature in the internal walls is still low.

This situation can cause the atomized fuel from the nozzles to stick to the cylinder walls—and from there, it’ll seep across the piston rings and enter the crankcase.

Poor maintenance

Fuel dilution, both in diesel and gasoline, is a gradual process. Usually, if diesel fuel slowly seeps into the crankcase, it will have little to no impact on the oil viscosity. But if it keeps seeping into the oil over a long period, it will build up and affect the motor oil.

But the good news is that you can prevent it from building by making regular engine oil changes. Ignoring these regular oil checks and changes will have adverse effects.

Aside from that, check the nozzles to ensure they are always in good condition, as faulty nozzles can send an incorrect amount of fuel into the cylinders. This will invariably cause inefficient combustion.

diesel in oil meaning

What are the symptoms and consequences of diesel fuel in oil?

Now you know what causes diesel fuel delusion, but you may still want to know diesel in engine oil symptoms and their consequences. Diesel in oil could be a common issue. It can affect your engine in three ways—by causing inadequate oil viscosity, lower addictive content in engine oil, and contaminating the motor oil.

Loss of oil viscosity

Motor oil lubricates reciprocating internal engine parts—preventing them from rubbing against each other, car overheating, and engine wear. The engine oil has to be at a certain viscosity to grease the internal engine components properly. When diesel fuel dilutes the motor oil, it becomes too light to lubricate the internal parts.

Driving with too runny oil is as bad as driving with low oil in the engine. Both situations will damage the engine in the long run and drop expensive repair bills on the table. Plus, if the diesel in the oil is too much, it’ll cause overheating and slight engine wear in the short term.

Lower addictive content

Another symptom and consequence of diesel fuel in oil is a reduction in addictive content. Fuel dilution in oil not only lowers the oil viscosity but also pulls down the addictive content in the oil.

Engine oils are formulated with several addictive, which form layers around the internal engine components. So, as the internal engine components reciprocate, the addictive layers on these components scrape against themselves instead of the metal components scrapping against other reciprocating metal components.

However, if diesel fuel enters the engine oil, the additives in the motor oil won’t have enough strength to form a protective layer around the moving engine parts. The absence of this protective layer can cause short-term or long-term problems when the diesel fuel and oil mix.

Oil contamination

Diesel and fuel are incompatible and are meant to run on separate compartments. As if loss of oil viscosity and lower addictive content are not enough consequences for diesel entering the crankcase, it can even cause a more concerning issue—it reduces oil effectiveness through oxidation.

Oxidation reduces the engine oil lifespan, and it happens naturally. However, when diesel enters the oil, it fastens the effect of oxidation. You can tell when oil is oxidized by its darkened color, engine sludge, and increased oil viscosity. Always change your motor oil once you notice these signs.

Oxidation reduces the lifespan of the motor oil, lowers the protective agents in the oil, and causes rust and corrosion within the engine. However, you can fix this problem by contacting a mechanic to track why diesel is mixing with the motor oil and fix it.

How to diagnose and fix the problem of diesel fuel in oil?

As I have pointed out above, the consequences of diesel in motor oil are lower addictive content in the oil, loss of oil viscosity, and oxidation. These will cause overheating, oil leaks, and a few other issues.

Confirm you have diesel in the oil

The first step is to confirm you have diesel in oil. You can do this either by running a diesel in oil paper test, smelling the oil for a diesel, or taking an oil sample for a test lab. The first two method are actually quite effective. However, the last method—running an oil lab test is the most effective method as it will say the percentage of diesel in the oil.

Check the injectors

Faulty injectors, bristled injector rubber sealing, and other injector-related issues are the common reasons why diesel enters the engine oil.

Check out for blocked diesel injector symptoms—misfiring cylinders, engine stalling at idle, rough idling, starting issues, and fluctuating rpms. Also, check for diesel injector seal leaking symptoms—bad fuel odor and leaks around the injectors.

You can check the fuel injector seals for tightness with an air compressor. Disconnect the return fuel rail and connect compressed air to it. Add small fuel into the rail before connecting the compressor so that in case of a leak, air will pass through and make bubbles.

Also, test the injectors on a work table to ensure they are working correctly. A suitable working injector nozzle should not spray too much or too little. Injectors that spray too much fuel are usually the cause of engine flood and diesel in oil. Replace the O-rings if they are bad, and change bad injectors.

Check the cylinder heads for cracks.

Cracks in the cylinder head are a severe issue that could lead to replacing the entire engine. I wrote a comprehensive article detailing how to fix holes in the engine block. If the crack is too small, you can weld it, but if it’s not, you should change the cylinder head.

Check the diesel fuel pump.

Inspect the oil seals in the diesel fuel pump. Sometimes, replacing it prevents diesel from entering the motor oil. If the seals are worn or bristled, replace them with new ones.

How to prevent diesel fuel from mixing with oil in the future?

Sometimes, diesel in oil doesn’t mean there is anything wrong with the engine. It could be that the engine is never getting to the average operating temperature, causing some amount of fuel to seep into the crankcase and remain there.

The diesel in oil dilution is caused by the mode of operation of the car, such as lots of cold starts and short distance driving.

The most feasible solution in this case is to take a long drive at least once in three or four weeks. Just drive for a couple of hundred kilometers. You might find it quite therapeutic. Just pump up your ride like a bat right out of hell.

This could allow your oil to come back mentally refreshed and purified by getting extremely hot and burning off the fuel that enters the crankcase. So if you are wondering, will fuel in oil burn off? Now you know.

And if you drive short distances, change the oil and the filter twice as often. That is the countermeasure here. When you change the old oil, you start the clock again on the dilution fraud. If you start driving normally again, you return to the regular oil change interval.

Keep the injector and nozzle rubber seals in good condition. As explained several times in this article, issues with the injector systems are the common causes of diesel fuel in the engine oil.

If you have a faulty injector that is over-fueling, it will keep diluting your oil until you replace it. But if you keep your injectors in good condition, you prevent any form of fuel dilution. Also, always keep the rubber seals in good shape.

Final words

No matter what causes diesel in oil, you should note one thing—mixing these two liquids will make the engine oil ineffective. If an oil lab confirms you have a certain percentage of oil dilution in the engine, track the root cause by going through the diagnosis and fix procedures above, or have your mechanic do the task.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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