NUT 1535960 - Caterpillar

3500 Family Of Diesel EnginesBackround
This information in this article is specific to the 3500 Family of Diesel Engines only. In this article, any reference to engines is a reference to the 3500 Family of Diesel Engines.
The 3500 Family of Diesel Engines 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 degree of fuel dilution. The degree of fuel dilution largely depends on the condition of the injectors and the application and/or loading of the engine.
It is this degree of fuel dilution that needs to be monitored and may sometimes be a high enough concern to be further investigated.
See Caterpillar's publication, "Oil and Your Engine", SEDB0640, for additional background and reference information.
How Much Fuel Dilution Is "Acceptable"?
The Caterpillar limit for "excessive" fuel dilution in ALL Cat engines is 4 percent. This includes the 3500 Family of Diesel Engines. If the amount of fuel in the oil reaches 4 percent, the oil should be changed and the source of "excessive" fuel dilution should be found and corrected. Logically, the lower the amount of fuel dilution - the better. The Caterpillar fuel dilution limit has been derived from a combination of internal testing, actual field experience, as well as an oil viscosity blending equation. The primary concern is oil viscosity. An example of this effect is a 5 percent fuel dilution of a new nominal SAE 40 oil will reduce it's viscosity to the next lower viscosity level (equivalent of SAE 30). Caterpillar experience indicates that fuel dilution below 4 percent does not reduce engine or bearing life.
Fuel dilution may be determined by SOS testing. Caterpillar employs ASTM D3828-A "Standard Test Methods for Flash Point by Setaflash Closed Tester" for the determination of fuel dilutions of 4 percent or above.
Possible Affects Of Fuel Dilution
The primary concern with "excessive" fuel dilution is maintaining a safe oil viscosity. Excessive fuel dilution (above 4 percent) can "thin" the oil to the point that reduced oil film thickness causes an excessive wear rate or failure.
The wear rate and failure risk from excessive fuel dilution will largely depend on the level of excessive dilution and the duration of operation. Very large amounts of fuel in the oil could possibly lead to "lack of lubrication" type failures - bearing failures, premature engine wear.
With lesser levels of fuel dilution (below 4 percent), 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.
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