Why Does Diesel Engine Oil Break Down?

Why can't oil last forever? Why do we have to change our oil so often?

Many vehicle owners have asked this question. Manufacturers, too, wish that it were possible to eliminate oil changes because making time for an oil change is probably a vehicle owner's number one inconvenience.

The problem is, oil breaks down and is no longer able to keep your engine operating as optimally designed. Hence the need for an oil change regimen.

When we first wrote about this topic last fall we shared how understanding  oil chemistry and composition helps us understand the limitations of engine oil. This is why OEMs have recommended drain intervals, and also explains why ignoring these can cause us to risk voiding warranties. 

But there are additional reasons oil deteriorates and can cause engine failure if ignored. Machinery Lubrication published a helpful article by Jim Fitch of the Noria Corporation titled "Four Lethal Diesel Engine Oil Contaminants" that addresses this issue of oil breakdown. The bottom line here is that there is considerable value in periodic oil analysis, as we have noted elsewhere.

What follows is an abbreviated summary of the four destructive forces that can damage the integrity of your oil, potentially leading to the catastrophic failure of your engine. A link to the complete article can be found at the end of this story.

The Four Lethal Forces

Glycol
Radiator fluid with glycol can get into the oil a variety of ways, from blown head gaskets to defective seals to cylinder heads. According to one study, glycol was found to be present in 8.6 percent of 100,000 diesel engine samples tested. 

Even a small amount of glycol can cause serious issue. A 0.4 percent glycol coolant in the oil can cause soot to coagulate, leading to a dump-out condition resulting in sludge, deposits and oil flow restrictions. The article cites numerous other issues and they are all bad.

Fuel Dilution
It is fairly common to have minor fuel dilution occur with frequent starts or excessive idling. Severe dilution (2 percent or more) indicates a more serious causes like leakage or fuel injector issues. 

In cold temps fuel dilution can cause waxing that results in low oil pressure or oil starvation. Fuel dilution can also thin out the oil so that it is no longer adequate to protect against wear in the combustion zone. Severe dilution undermines the effectiveness of oil additives. These and other issues are amplified when using a biodiesel lubricant.

Soot
Soot is a by-product of combustion that is present in all diesel motors. The presence of soot is normal, but when present in abnormally high quantities it is indicative of other problems that need to be addressed.

Combustion efficiency is directly related to soot production. Things to look at include poor ignition timing, excessive ring clearance or a plugged up air filter. 

Fitch details a host of other concerns that can crop up when you have soot issues inside the engine. 

Water
Though we can't live without water, it is also quite destructive, especially inside your engine. As the saying goes, oil and water don't mix. Too much water will result in that greasy slop that is not going to be fixed with an oil change. It also increases the corrosive power of acids inside the engine and attacks additives. 

* * *

The article closes by noting that total engine failure is something that happens over time as a result of the cumulative effect of one or more of these forces. This is not, the author notes, the some total of all that can go wrong in an engine. Dozens of other factors may lead to what seems like a sudden death. Usually, however, there are all kinds of warnings that we put off addressing or fail to notice.

Oil analysis is your best early warning system, though some of the signs of trouble can be observed as you do routine maintenance. 

Links

Four Lethal Diesel Engine Oil Contaminants

Champion Brands Offers Free Oil Sampling Kits for Diesel Engines