Diesel Fuel Dilution: Causes, Symptoms, and Prevention

Diesel fuel dilution, also known as diesel in oil, is a common issue that affects diesel engines. It occurs when diesel fuel leaks into the engine oil, causing an increase in oil levels. This can lead to engine wear and tear, and it is one of the reasons why regular oil testing is necessary. In this article, we will discuss the causes, symptoms, and prevention of diesel fuel dilution.

Causes of Diesel Fuel Dilution

Diesel fuel dilution can occur due to various reasons. One of the most common causes is faulty fuel injectors. Over time, the injector nozzles’ seals can wear out or become damaged, allowing diesel fuel to enter the engine oil. The number of faulty injectors can affect the amount of diesel that enters the oil.

Another cause of diesel fuel dilution is a malfunctioning exhaust gas regeneration system. If the diesel particulate filter (DPF) is clogged, diesel fuel can seep into the oil pan. Additionally, engine modifications that affect how the injectors spray fuel into the cylinders can also cause diesel fuel dilution.

Symptoms of Diesel Fuel Dilution

Diesel fuel dilution can have several consequences, including loss of oil viscosity, lower additive content, and oil contamination. When diesel fuel dilutes the motor oil, it becomes too light to lubricate the internal parts properly, leading to engine wear and overheating. The additives in the motor oil form layers around the internal engine components, preventing them from rubbing against each other. However, if diesel fuel enters the engine oil, the additives won’t have enough strength to form a protective layer, leading to engine damage.

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Diagnosing and Fixing Diesel Fuel Dilution

To diagnose diesel fuel dilution, you can either run a diesel in oil paper test, smell the oil for a diesel odor, or take an oil sample for a lab test. The most effective method is to run an oil lab test, which will give you the percentage of diesel in the oil. If you confirm that you have diesel in your oil, you should check the injectors for blockages, bristled injector rubber sealing, and other issues. You can check the fuel injector seals for tightness with an air compressor and test the injectors on a work table to ensure they are working correctly. If the injectors are faulty, replace them.

Prevention of Diesel Fuel Dilution

Regular oil changes can prevent diesel fuel dilution from building up in the engine oil. It is essential to check the nozzles regularly to ensure they are in good condition. Faulty nozzles can send an incorrect amount of fuel into the cylinders, causing inefficient combustion. Additionally, taking long drives once in a while can help burn off any fuel that enters the crankcase, preventing dilution.


Diesel fuel dilution can have serious consequences for your engine. It is essential to diagnose and fix the problem as soon as possible to prevent further damage. Regular oil changes, checking the injectors and nozzles, and taking long drives can help prevent diesel fuel dilution. If you suspect diesel fuel dilution, consult a professional mechanic for a proper diagnosis and repair.

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