LMM Duramax and Oil Bypass Filters

Volume 2 Issue 1 - Diesel Articles

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LMM Duramax and Oil Bypass Filters
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The LMM Duramax is certainly a step forward in its reduction of emissions into our environment. These controls, however, come at a cost. In Learning to Love the LMM Duramax Emissions System, Joel Paynton looks at what the LMM accomplishes and what it costs in terms of convenience and fuel economy. In this article, I want to take a quick look at what recycled soot does to the engine oil in the LMM and recommend bypass oil filtration as a worthwhile protection for this considerable investment.

Oil bypass filters for large diesel engines are accepted as a necessity and have been recommended by several aftermarket filter companies for many years. As a certified lubrication specialist, I have recommended bypass filtration systems as a solution for many diesel applications, though not for every application. Prior to the LMM, the Duramax engines were capable of dealing with soot. Even if they could benefit from a bypass system, they could certainly get by without one. Not so with the new LMM. I absolutely recommend bypass filtration for this engine.

What is Bypass Filtration?

Bypass filtration refers to systems in which a portion of the oil pump output, usually no more than ten percent, is diverted to an auxiliary filter. The system is so-named because in a conventional bypass system, the oil bypasses the engine and returns to the oil pan without providing lubrication to the engine. In fact, some reference manuals refer to such systems as parasitic filtration systems. It is important to note that while the appellation is not totally without merit, bypass systems do not divert enough oil from the engine to fall below GM’s specifications. There is also a second type of bypass system in which none of the oil is diverted from its flow through the engine - more on that later.

Oil Bypass Filters Flow Diagram

A typical single remote (parasitic) bypass filtration system diverts a fraction of the oil in order to remove smaller particles that the full flow filter misses.

The need for a bypass filtration system arises when the filtration provided by the stock, full flow oil filter is insufficient to remove enough of the oil’s contaminants. Because the standard filter must allow sufficient oil flow to the engine to keep it properly lubricated, the filter media inside are, by design, thin enough to allow the oil to flow relatively easily. Of course, it would be no good to make the standard filter media more dense – so that it could handle smaller particles – if this meant that the oil was hindered from getting to the engine. Nobody wants a molten mass of metal under the hood, no matter how clean it might be. This means that standard oil filters, even good ones, can not deal with particles in the oil that are smaller than about 15 microns. A bypass filtration system, on the other hand – by only filtering approximately ten percent of the oil at a time and leaving the other 90 percent to do the lubrication work – complements the standard, full flow oil filter and allows the overall lubrication system to both provide sufficient lubrication and filter smaller particles down to the three-micron range. Even though the bypass system only deals with a fraction of the oil on any given pass through the system, over time, the complete volume of oil in the vehicle is treated by the finer media in the oil bypass filters.


The tight clearances in modern engines like the LMM can be affected by abrasive particles as small as five microns. Since full-flow oil filters do not remove particles smaller than 15 microns with any consistency, a bypass filtration system will protect any engine better than a stock filter alone.


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