Lubricating Oil Dilution

To completely understand the detrimental effects of oil dilution by either fuels or coolant there are a few fundamental concepts that need to be understood.

Viscosity in its simplest definition is a cohesion of particles which offers resistance to flow. It is a fluid friction within its own molecular structure (see illustration). More technically, it is the characteristic which allows the fluid to maintain shearing stress resistance dependent upon velocity of flow at a given orifice dimension.

Temperature is the greatest contributing factor to viscosity change. Viscosity will decrease with higher oil temperature as will strength of the lubricating oil film. Dramatic temperature rise will result in reduction of oil film thickness and may cause total oil film breakdown as well, both of which will allow metal to metal contact of components, and ultimately failure.

Gasoline and engine coolant in the lubricating oil will create a self-destruct situation for not only the bearings, but the piston rings and cylinder walls as well. Confirmed evidence of fuel dilution, by oil sample, can often point out suspect components during failure analysis. For example, cylinder walls and piston rings showing excessive wear prior to a bearing failure will indicate that an over-fueling (cylinder wash) has occurred. Bearing failures with minimal cylinder wear indicate that fuel dilution was introduced directly into the lubricating oil, such as through a fuel pump.

Coolant in the oil creates a froth, the lightened color of the lubricating oil looks almost like a "milk shake". This condition can create an oil starvation problem and will most often result in bearing failure. Vehicles that show this indication only on the underside of rocker covers and the filler caps need to be addressed with caution. Driving history becomes a critical factor in the diagnosis of these engines. Consistent short interval driving will often cause a condensation condition (particularly evident in aluminum rocker covers) that can cause a misdiagnosis of coolant in the oil. Verification of oil dilution by the coolant should always be verified with an oil sample test.

In the case of diesel engines (due to their unique combustion process) there is a small amount of fuel dilution that occurs during normal operation. Monitoring of the amount of fuel dilution by oil sampling is often the criteria which determines oil change intervals. Excessive amounts of diesel fuel in the oil will also create a potential failure situation for numerous engine components.

The AERA Technical Committee