Question 200Motor Plants - Assistant Engineer

The correct answer is C) diluted with fuel oil. A decrease in the flash point of diesel engine lube oil indicates that the lube oil has become contaminated or diluted with a lower flash point substance, such as fuel oil. When lube oil is diluted with fuel oil, the flash point of the mixture decreases, as fuel oil has a lower flash point than the lube oil. This is an indication that the lube oil has been contaminated and needs to be changed. The other options are incorrect because: A) Contamination with sludge would not necessarily cause a decrease in flash point. B) Contamination with carbon would not cause a decrease in flash point. D) Dilution with water would cause the flash point to increase, not decrease.