Four Lethal Diesel Engine Oil Contaminants

Jim Fitch, Noria Corporation
Tags: contamination control, fuel dilution, water in oil

Some contaminants are important to monitor and analyze because they are root causes of premature oil degradation and engine failure. Other contaminants are symptomatic of an active failure condition that requires a response other than just an oil change.

For instance, seal damage leading to fuel dilution or glycol contamination cannot be remedied by performing an oil change or switching to a better quality lubricant. Such symptom-based contaminants are also root causes that enable new failures to occur. The value of oil analysis in detecting problems early goes without saying.

Diesel Engine Oil Contaminants

Any one of the contaminants described below is capable of causing premature or even sudden engine failure. I've left dirt contamination off the list because I covered particle-induced engine failures in a previous column.

It is worth noting that problems are more pronounced when contamination combos exist, such as high soot load with glycol or high soot load with fuel dilution. There are numerous failure pathways and consequential sequence of events. Thousands of diesel engines fail prematurely each year aided by the presences of glycol, fuel, soot and water in the engine oil.


Glycol enters diesel engine motor oils as a result of defective seals, blown head gaskets, cracked cylinder heads, corrosion damage and cavitation. One study found glycol in 8.6 percent of 100,000 diesel engine samples tested. A separate study of 11,000 long-haul trucks found severe levels of glycol in 1.5 percent of samples and minor amounts of glycol in 16 percent of samples. The following are some of the risks associated with glycol contamination:

Fuel Dilution

Frequent starts of an engine, excessive idling and cold running conditions can lead to moderate fuel dilution problems. Severe dilution (excess of two percent) is associated with leakage, fuel injector problems and impaired combustion efficiency. These are symptomatic of serious conditions that cannot be corrected by an oil change. According to one reference, 0.36 percent of total fuel consumption ends up in the crankcase. Problems associated with fuel dilution include:

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Soot is a by-product of combustion and exists in all in-service diesel engine motor oils. It reaches the engine by various means of blow-by during engine operation. While the presence of soot is normal and expected for a given number of miles or hours of service on an engine oil, the concentration and state of soot may be abnormal, signaling a problem with the engine and/or a need for an oil change. Following are some issues related to soot contamination:


Water is one of the most destructive contaminants in most all lubricants. It attacks additives, induces base oil oxidation and interferes with oil film production. Low levels of water contamination are normal in engine oils. High levels of water ingression merit attention and are rarely correctable by performing an oil change. The following are some additional notes on water contamination:

Failure Development Period

The failure development period can vary considerably for these contaminants. Most sudden-death failures from moderate levels of contamination will usually have one or more aggravating factors (the combo effect). Conversely, massive concentrations of one or more of these contaminants can result in sudden-death failures unaided by an aggravating circumstance. There are dozens of other aggravating factors that can drastically shorten the failure development period as well. More typical is when a moderate problem goes unnoticed and develops over time. This can shorten engine life from say, 750,000 miles to 300,000 miles.

The cumulative effect of oil contamination on engine reliability, fuel economy, exhaust stream emissions and maintenance cost of a large fleet is massive. There are no motor oil additives that control the damage caused by these contaminants. Therefore, proactive maintenance and oil analysis are critical strategies to counteract risks.