Fuel Dilution in Diesel Engines: Causes, Signs, and Prevention

Have you noticed that your engine's oil levels appear higher than usual lately? You’re also catching a distinct smell of diesel from the dipstick when you take it out.

What could be the problem?

Unfortunately, this means it’s time to check the condition of your vehicle’s engine, specifically its oil.

In this article, we’ll look at some of the most common causes of fuel dilution and how to spot the telltale signs. We’ll also explain what you can do to protect your vehicle.

What is Fuel Dilution in a Diesel Engine?

Fuel dilution is a critical issue that occurs when diesel fuel breaches the crankcase and mixes with engine oil. This significantly reduces the oil's viscosity by disrupting the normal formation of the resilient lubricating film that protects your engine. As a result, the wear and tear of all the moving parts within the engine, including rings, pistons, and cylinders, increases dramatically.

Causes and Contributing Factors

So, what causes fuel to mix with oil?

Some of the most common contributing factors include:

• Leaking or dirty fuel injectors: If your engines’ fuel injectors are stuck open, excess fuel can leak into the affected cylinder.

• Prolonged idling and frequent short trips: Your engine may not reach optimal operating temperature if you often have to leave your engine working while waiting for someone or don’t take the occasional long trip. This prevents complete fuel combustion.

• Starting your engine in cold conditions: Besides idling and short trips, starting your car in cold weather can inject excess fuel into your engine. The extra fuel can wash oil from the cylinder walls, resulting in damage to various engine parts.

• Blow-by: Worn piston rings allow unburned fuel to breach the crankcase.

How to Identify Signs of Fuel Dilution? 

Understanding fuel dilution issues is an essential part of owning a diesel engine. Because they can cause severe damage to your diesel engine, such as:

• Wear of crucial parts (rings, camshafts, bearings, and pistons)

• Poor engine performance and overheating problems

• Corrosion of engine components

• Blocking of oil passages and filters

• Higher maintenance and repair costs

Using detergent additives can further exacerbate the problem because the oil’s ability to prevent deposits is reduced. This usually leads to greater oil consumption, additional top-offs, and unnecessarily frequent oil changes.

That’s why being able to identify the telltale signs is crucial. So, if you suspect your car suffers from a fuel dilution problem, be wary of the following symptoms:

• Significant reduction in the oil’s viscosity

• Strong fuel smell when inspecting the dipstick

• Increased oil levels

• Engine misfires or noticeable bad engine performance

• Dramatic rise in fuel consumption

• Strange engine noises or an increase in exhaust emissions

• Increase in the quantity of burned diesel fuel traversing the piston rings 

Detecting any of the above-listed signs during maintenance is a good reason to take your car to the mechanic and have it inspected before any critical engine damage occurs. Taking proactive measures will undoubtedly save you both time and money in the long run.

How Can I Prevent Fuel Dilution Issues?

If you want to mitigate the chances of fuel dilution problems in the future, some of the simplest prevention methods include:

• Reducing the number of short trips: Adjusting your travel to longer intervals will allow your engine to operate at its optimum temperature. This is a good opportunity to plan your next out-of-town trip!

• Minimize idling by turning your engine off after it has cooled down. Use high-quality synthetic engine oil with proper viscosity: Always check your service manual to ensure you’ve picked the right one. This will guarantee a better guard against accelerated engine wear.

• Monitor your engine oil regularly to baseline your engine’s fuel dilution rating.

What is Fuel Dilution Rating, and How Can I Measure It?

Fuel dilution rating shows the fuel in your engine’s oil. And it can be affected by many factors. For instance:

• Over-fueling

• Prolonged idling

• Flawed injector tips

• A high air-to-fuel ration

• Irregular ring seating

• Engine timing problems

Regular oil analysis and maintenance can help you detect and address fuel dilution and other underlying issues. The best way to measure your rating at home is by using an oil analysis kit, such as Fleetguard’s CC2525 Oil Analysis Test Kit. This Lubricating oil analysis tool offers a complete overview of your engine’s health status and the amount and type of contaminants in the oil.

Remember that some mixture of fuel and oil is to be expected. As long as the oil's viscosity doesn’t change too much and it continues to lubricate the engine properly, there shouldn’t be any causes for concern. 

However, if your fuel dilution rating exceeds the acceptable 3.4% for diesel engines, you should consult your mechanic as soon as possible.

Watch this step-by-step guide on pulling and submitting fluid samples using the Fleetguard CC2525 Oil Sample Kit.

Embed the following video:

https://youtu.be/9NcFt8HeSko?si=zQ96Rxihx4tBBakg 

What is the Cummins Oil Sample Kit CC2525 kit from Fleetguard?

The Fleetguard Atmus Filtration Oil Sample Analysis Kits provide all the details you will need to determine the overall condition of your engine oil, with test results ranging from standard to severe. 

The kit includes the following tests to evaluate engine wear and contamination:

• A Spectrograph Analysis of the 24 metals used to build an engine: Establish the levels of wear metals, silicon, and additives in your engine’s oil. 

• A Fuel Dilution Rating: Measure the quantity of burned diesel fuel that has traversed the piston rings.

• A Soot Contamination Rating: Gauge the amount of solid particles present in your oil. Soot results from diesel engine combustion, made up of carbon residue from fuel, air, and moisture after ignition. If excessive soot is produced and additives cannot contain it, deposits will develop on the rings, weakening the seal between the pistons and cylinder liners. By assessing the percentage of soot in your oil, you can proactively address necessary repairs before they cause potential failure.

• An Oil Analysis Kit: Check for the presence and content levels of metals and other elements, such as aluminum, chromium, iron, copper, lead, calcium, and more. Knowing the levels of these materials in your oil can help you identify wear patterns in a specific part of the engine. High amounts of worn metal usually indicate abnormal wear or corrosion problems.

• A water contamination test: Assess the percent of water contamination in the lubricant.

Moreover, the Oil Sample Kit analysis can measure the levels of insoluble matter like carbon, fuel, and dirt. These indicate the rate at which your oil is oxidizing and receiving contaminants and how effectively the system's oil filtration is functioning. 

The kit also helps determine the oil’s viscosity, alkalinity, and acid levels. It even provides information on whether the proper levels of detergents and anti-wear additives are present in the engine. 

All Fleetguard Oil Sample Analysis Kits are sent for analysis to ALS, a global testing service. The test results are then relayed to the email address provided on your sample kit.

Don’t risk getting stranded on the road because of engine failure. Protect your vehicle by promptly identifying potential serious issues and ensuring your road safety! 

FAQs of Fuel Dilution In Diesel Engines

1. Why is fuel dilution harmful to diesel engines?

Fuel dilution reduces the oil’s viscosity, which affects its ability to lubricate the engine properly. This leads to increased wear and tear, corrosion, and potential engine failure.

2. What percentage of fuel dilution is acceptable?

For diesel engines, fuel dilution levels should not exceed 3.4%. Higher levels indicate severe contamination and require immediate attention.

3. How often should I test my oil for fuel dilution?

It’s recommended to test your oil during routine maintenance or if you notice signs like a strong fuel smell or increased oil levels.

4. Can fuel dilution be fixed?

Yes, addressing the root causes—like faulty fuel injectors, blow-by, or frequent short trips, can prevent further fuel dilution. Regular oil changes and maintenance also help.

5. Where can I buy the Fleetguard CC2525 Oil Sample Kit?

You can purchase the Fleetguard CC2525 kit from KLM Performance.