YOKE AS 3586189 - Caterpillar

How Much Fuel Dilution is "Acceptable"?
The Caterpillar limit for "excessive" fuel dilution in ALL Cat engines is 4 percent. This includes the 3500 Diesel Engines in Caterpillar machines. 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.
SOS would be the best method for determining if excessive fuel dilution is resulting in accelerated wear.
Application Or Loading Of The Engine
In both, internal and field tests, normal, properly functioning engines can exhibit "excessive" fuel dilution from idling for long periods at a time. A test engine was run on a test stand at 1800 rpm continuously, unloaded. After 250 hours, the fuel dilution was 6 percent, and after 1000 hours, the fuel dilution was 24 percent.
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.
Potential Sources Of Fuel Dilution
In 3500 Diesel Engines in Caterpillar machines, fuel dilution of the