Soot Happens

Excessive soot levels in oil can cause engine issues

Furthermore, soot loading also causes a viscosity increase, which impedes oil flow and increases engine wear leading to premature engine failure, add Lubrizol officials. This is especially the case in cold weather starts where a soot-laden engine oil can increase the time it takes for the oil to reach critical engine parts, such as the valvetrain.


The tendency for soot particles to aggregate or join together is called agglomeration, explains Betner of Citgo. Soot agglomeration increases when the oil can no longer handle or disperse the level of soot load in the oil.

Soot particles are very small in size and generally pass through the filter media until they begin to agglomerate, he says. At this state, the oil condition can cause several problems:

  • Oil thickening or viscosity increase. The accumulation of excessive soot leads to oil thickening which can cause poor oil flow during engine start-up and reduce lubrication protection.
  • Oil filter plugging. As soot accumulates and begins to agglomerate, the oil filter will collect more soot and eventually reach filtration capacity. Once the oil filter has reached capacity, the engine demands oil either filtered or unfiltered.

The result, Betner says, is that the filter is bypassed to insure the engine gets oil. When this condition occurs, the engine may show a warning signal.

  • Higher engine wear. Soot is carbon, and as carbon agglomerates and accumulates it becomes more abrasive. Equipment maintainers who perform oil analysis can see the result of the increased soot loading in several ways. When the percentage of soot in the oil increases, this can result in an oil viscosity increase. If allowed to accumulate to higher levels, the engine wear metals will also increase.
  • Increased oil consumption. As oil increases in viscosity due to higher levels of soot loading, there is a tendency for the greater amounts of the thickened oil to accumulate on the engine cylinder wall. As the engine piston moves upward, the increased accumulation of soot-laden oil on the cylinder wall may result in the excess oil being released into the combustion process. This condition can result in increased oil consumption.

Some equipment operators have reported increased oil consumption in the later stages of the engine oil service interval, notes Betner. This, in part, can be attributed in some cases to the increased level of soot as the oil service interval increases. Oils that have greater ability to disperse soot tend to reduce the increased oil consumption during these stages of the oil service interval.


Betner recommends taking the following measures to help mitigate the problems associated with soot loading in the oil:

  • Make sure the engine fueling system is maintained properly and the fuel injectors do not cause improper fuel injection.
  • Provide operator training for both vehicle operation and observation of engine warning systems.
  • Make sure the engine electronic system is properly calibrated and/or updated as needed.

Also consider utilizing a reliable oil analysis program, he adds. A comprehensive oil analysis program, such as the Citgo LubeAlert HD, will show the level of soot, the oil viscosity and the engine wear levels.

Knowing these factors allows equipment maintainers to gain better insight as to the oil condition, the impact of oil change intervals and the overall impact on the engine and filtration system, says Betner.

One more step in ensuring good engine performance is regular preventive maintenance and the use of high-quality engine oils formulated with the latest additive technologies for handling the soot produced in modern engines. Be advised, there are differences in engine oil quality when it comes to handling higher levels of soot.

By way of example, Citgo includes a “very aggressive” Soot Arrest Technology in its premium CITGARD heavy duty engine oils, says Betner. This engine oil technology has been designed to handle the different type of soot produced since the introduction of exhaust gas recirculated engines. These engines produce soot that is more difficult to disperse.

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