The Black, White and Greys of the Duramax Fuel Filter - 2

Volume 1 Issue 2 - Diesel Articles

Article Index
The Black, White and Greys of the Duramax Fuel Filter
Let's Blame Fuel Quality
Three Words: Maintenance!
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Let’s Blame Fuel Quality

Nearly coincidental to our experiment, an internal preliminary information document gave us some key, though slightly misleading information: if you are experiencing a multiple-injector failure and you suspect fuel quality, install a pre-filter (GM had already set up a Racor-Parker kit). The misleading part about this document was that it tried to set the focus of the injector failure issue on the always nebulous area of fuel quality rather than on the DMax fuel filter inadequacy itself. Theoretically, you could use perfectly refined diesel fuel and not have a problem; but refineries never perfectly separate the various fractions of crude oil from each other. The reality was that I could not find any obvious fuel quality or contamination issues with most of the trucks that I was replacing injectors on.

Time passed. The trucks on which we installed pre-filters ran fine. If one of them did experience an injector failure, there was a very obvious and atypical fuel contamination issue to blame. Armed with this empirical evidence, we concluded that fuel contaminants that should be caught by the system’s filter were the cause of the Duramax’s high-tech injectors’ demise.

Double Teamed!

More bulletins followed which, again, again promoted the availability of the pre-filter kit and emphasizing the importance of adhering to fuel filter maintenance intervals. It wasn’t until I attended a course on the Duramax, however, that I learned what was actually happening internally in the fuel system to cause the injectors to fail. Sure enough, the fuel filter stood at the center of the storm.

Two things that I mentioned earlier now come into play. The first is that water is not the only contaminant found in the fuel you and I buy at the pump – even if it is the primary culprit when it comes to actually damaging the injectors. Fuel also contains asphalt residual, called asphaltenes, that the refining process (the second thing) fails to completely remove when crude oil is broken down to its component parts or “fractions” as they are called. Asphalt is a heavy fraction that we all know and love as the substance used to hold aggregate together on roads and such. However, some asphaltenes are always undesirably present in diesel fuel.

Back to the fuel filter. During normal operation, the pores in the media of the Duramax fuel filter become clogged with the omnipresent asphaltenes. As more pores become clogged, the fuel has less and usable media through which to flow. Restricted flow increases fuel flow velocity. Now remember that the DMax fuel filter doubles as the water separator. Normally, the tiny particles of dissolved water present in the fuel are too large to pass through the filter. They coalesce on the surface of the filter and run down to the bottom of the can where the water-in-fuel sensor resides. However, if the asphaltenes have plugged enough of the pores in the filter, resulting in a higher fuel flow velocity across the filter, those same water particles can stick to the filter media. If they stay there long enough, the filter media swell creating, one by one, larger pores that will now allow the water particles – we could call them injector assassins now – to pass through. What happens next? You guessed it: fuel injector homicide!

Looking at the photos (above and next page) of the fuel filters that have reached or exceeded the maintenance interval, it should be obvious why replacing the fuel filter consistently before too much asphaltene restriction occurs is imperative. Accordingly, GM stipulated a replacement interval of every 15,000 miles / 24,000 kilometers. If the OEM fuel filter had been doing everything it should have been doing prior to being neutralized by the asphaltenes – something that should not occur if a fresh filter has been installed at the proper service interval – this would have been the end of the story. Unfortunately, it is not the end of the story. The original design, single stage fuel filter was rated for five microns. (To give you an idea of the size of five-micron particle, it takes 5,080 such particles lined up end-to-end to equal one inch). The filter, however, allowed too-high a percentage of particles larger than five microns to pass. This anomaly occurred because manufacturing filter media, like refining oil, is not a perfect process: it is not feasible to manufacture a filter where every pore is exactly five microns. So there will always be a percentage of larger particles that make it through the filter. These particles include dissolved water as well as the asphaltenes themselves. They can certainly be a contributing factor to injector failures. Research reported by sources such as Heavy Duty Trucking in an article entitled “Finer Filtration: Is it the Answer” (October 2003, page 86) and “Your Engine’s Tiny Problem” (October 2003) indicate that particles larger than seven microns can cause excessive wear in a high pressure fuel system such as the one in the Duramax. Both of these sources reference SAE Paper 980869 and Detroit Diesel Engine Requirements Manual 7SE70 0209. I will note here that the use of fuel additives that contain water emulsifiers increase the likelihood that dissolved water will succeed in getting through the filter – be cautious about using them.

That leads us to the reason the pre-filter solved the problem. Suddenly the main filter had a much more manageable task: all it had to do was catch the small fraction of larger particles that made it through the pre-filter. The percentage of water and other contaminants making it through to the injectors dropped dramatically. The presence of a good pre-filter along with proper maintenance in keeping with the main filter service intervals generally allowed the early Duramax injectors to run quite happily.

The Solution? Add Another Element

Another affirmation of our hypothesis that the original OEM single-element fuel filter was the cause of the failing injectors is that GM subsequently redesigned the filter. In fact, it has had several updates. The first major update was arguably the most important: GM went to a dual element, essentially packaging a pre-filter and a main filter in one can. Then they gave it more capacity by making it longer. Also important, the filter media themselves were improved. Although the original filter was rated at five microns, reliable internal sources have told me that it did not quite live up to that specification. Finally, GM drove home the fact that the original DMax filters were sub par with a (now expired) campaign to give customers a free fuel filter replacement – with a new dual element of course!

Joel Paynton is an award-winning GM technician who specializes in Duramax fuel systems. He also does custom programming for any GM powertrain. Visit him on the web at


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