March 12, 2020

Fuel Dilution: Example, Effects and Recommendations

By Jeff Walkup

You live in a world of information overload combined with analysis paralysis. What you need is actionable insight that guides you on what to do and when.  Each moment wasted on chasing down and putting out fires directly effects your bottom-line.

That’s why it’s important to partner with a reliability company that can take data and turn it into action. As with anything in life, you get what you pay for – it’s the same in the reliability world.  As a fluid analysis provider, one of the many things Fluid Life does is take data and turn that into recommendations and corrective actions to achieve real world results. We go beyond the “cookie cutter” model as shown in this simple example of a sample anomaly.  

Fuel Dilution: An Example

The engines where the samples were taken from have a unit injector design, which requires lubrication at the upper end of the plunger and barrel. Lubrication is needed to minimize plunger and barrel wear, as well as to minimize possible seizures. This lubrication of the plunger and barrel comes from the fuel, which must reach the upper end of the plunger and barrel. In doing so, some of the fuel leaks to the crankcase. Therefore, it is normal and expected for these engines to have some fuel detected in used oil samples. The degree of fuel dilution largely depends on the condition of the injectors and the application and/or loading of the engine in question. For this particular OEM, the limit for “excessive” fuel dilution in engines is 4%.

If the amount of fuel in the oil reaches 4%, the oil should be changed and the source of “excessive” fuel dilution found and corrected. However, an oil change does not locate and correct root cause; it merely addresses a symptom.

In this example, the sample of record was taken on 1/27/2020. The sample showed fuel being detected at 17.28% with a corresponding reduction in oil viscosity at 9.06cSt at 100C. As viscosity is considered a lubricants number one property, this suggests considerable fuel dilution taking place. This sample was flagged and an alert issued to the customer along with actionable recommendations.

Fuel Dilution: The Effects

  • The primary concern with “excessive” fuel dilution is maintaining a safe oil viscosity. Excessive fuel dilution (above 4% on this particular make- model) can “thin” the oil to the point that reduced oil film thickness causes an excessive wear rate or failure to take place and excessive and consecutive events are cumulative.
  • Very large amounts of fuel in the oil could possibly lead to “lack of lubrication” with the known type failures i.e. bearing failures, premature engine wear.
  • With lesser levels of fuel dilution (below 4%), these engines should run satisfactorily to normal overhaul with no noticeable or measurable side effects. In addition, these engines are known to have very durable crankshafts and main bearings; thus making these engines less susceptible to excessive fuel dilution/lack of lubrication failures than many other engines.
  • Under high loads and the resulting high crankcase temperatures, some fuel is removed from the engine oil by distillation (or “burned off”). Therefore, an oil sample taken after a long, highly loaded period may indicate less fuel dilution than a sample taken after periods of idling or low loads. Also, fuel dilution may increase in the winter months as the engine oil temperatures may run cooler and/or engines idle longer to keep the engine warm. Engines running “cold” due to leaking thermostats may also exhibit an increase in fuel dilution.

Three possible areas where typical fuel dilution may originate

  • The first and most likely area would be the unit injector itself.
  • The second but less likely will be the fuel transfer pump.
  • The third and least likely cause could be a porous cylinder head.

Recommended maintenance / corrective actions

  • If fuel dilution exceeds 4%, the source of the fuel dilution must be found and repaired. The engine oil must also be changed when fuel dilution reaches 4% and consecutive samples/ trends need to be monitored.
  • The transfer pump weep hole should be inspected periodically and cleaned if necessary to ensure it remains open.
  • An engine idling extensively should be brought to full load and run at operating temperature to help not only to “distill off” the fuel, but to also help rid the oil of some of the condensations built up from running an engine “cold”.
  • Should fuel dilution continue to increase “Conduct Dye Check for Leaking Fuel Injectors as per OEM Spec”. Use a black light to inspect all the unit injectors around the rack bar area. A leaking injector will show a definite streak (noticeable difference) of dyed fuel from the rack bar area. ALL injectors will seep and may show a slight amount of dye during this test. Some seepage is considered to be NORMAL
  • Replace and adjust injectors that are significantly leaking (not just seeping).
  • Reconnect fuel lines, install valve covers and run engine to check for oil/fuel leaks.
  • Change engine oil and resample after 100 hours of operation in order to confirm corrective actions have solved the anomaly as noted by used oil analysis. Having an extensive sample database in myLab lets you trend against a known segment with similar anomalies in comparison to others in the same operational and environmental regime. Advanced algorithms, artificial intelligence and machine learning can then be overlaid with the skilled and trained human element. 
  • Track and record corrective actions completed in myLab. This brings clarity to the Fluid Life CARE Team and accountability to you as the end-user.


With 30+ years of experience and nearly 10 million oil sample results in our database, our team of reliability experts can help you navigate the world of information overload and eliminate analysis paralysis. We can wade through the data and provide recommendations that let you take action. We understand the difference between testing, analyzing the data and then applying engineering domain knowledge with practical real world experience to make the process actionable and valuable for you our valued customer.  Contact a Fluid Life representative today and get started.