The Black, White and Greys of the Duramax Fuel Filter

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!
All Pages
Grandpa operating Case 1150B series dozer with scraper

Grandpa used to lecture me about the importance of draining the water separator on his farm’s diesel tractor. This man, who wouldn’t hesitate to fix things with haywire or binder twine was, however, a purist when it came to maintenance. Experience had taught him that draining the water separator could mean the difference between a well performing engine and one with problems – or one that didn’t run at all.

The investment required to properly maintain the fuel system meant avoiding the corrosion and scoring caused by water when it runs amok in the fine-tolerance components of a diesel engine.

Duramax engines are significantly more sophisticated than Grandpa’s tractor. One might think that these more sophisticated engines can look after themselves a little better. In a sense that is true: today, we have more feedback than ever coming from our engines. But to think that these engines can handle not being maintained as well as their less sophisticated forerunners is far from the truth. In fact, just the opposite: today’s fuel injectors and other components require much higher tolerances. Consider that a typical conventional diesel fuel system prior to the DMax operated at fuel pressures of 1,200 to 1,500 PSI. Then compare those numbers to the original LB7 fuel injector that operated in the range of 4,500 to 23,200 PSI!

What’s more, the DMax injector has to seal tight at these pressures unlike the injectors of conventional, distributor injection pump systems. Additionally, the LB7 injector was easily 10 times faster than its conventional counterparts – the response time from when the injection control module energized the injector to when it began delivering fuel was an incredible 150 millionths of a second and it was capable of two injection events per cycle. The newer DMax injectors are even faster and operate at higher pressures. They are capable of up to five injection events per cycle and run up to 26,500 PSI. Really, the performance requirements of the new common-rail diesel injectors absolutely blow away any conventional distributor injection pump diesel fuel systems. The result of much higher precision, operating pressures and speeds is that modern fuel systems are much more intolerant to the fuel contaminants – including water – that we will discuss in this article. These engines must be properly maintained or they suffer the consequences. Knowing this, GM engineered a filtering system up to the task, right? Heavy equipment and highway tractors typically feature multi-stage water separators and fuel filters. The DMax, with its super-fine tolerance injectors – and smaller internal components than a common-rail injector on a highway tractor – would possess a similar design, right?. Actually, and as you probably already know, the DMax engine includes only one relatively small filter that pulls double duty as the system’s water separator. This combination, sophisticated equipment and a single-stage fuel filter, was a formula for potential problems. Mix in Murphy’s Law and potential problems become real life bring-it-into-the-shop problems. The good news is that we now understand the source of the problems and GM has made significant improvements to the DMax fuel filter.

Duramax diesel fuel injector tolerances

Kentworth two-stage fuel filter

(Click on either image to get more information or enlarge)

Too Many Failed Duramax Injectors

For a while, the diesel techs at the dealership were changing Duramax injectors on a daily basis. We would see five or so trucks lined up waiting for those precious injectors. It didn’t take long to start researching why they were failing so often. We discovered quite early on that if one injector was failing, the other seven were right behind it. Either Bosch, who had been mass producing common-rail injectors since 1997, was consistently turning out junk or something was killing these injectors.

I thought the latter possibility made more sense. Other manufacturers, Mercedes Benz for instance, had been using the injectors for a few years by that point without such pervasive issues. Surely Bosch had it figured out? The hypothesis that something was prematurely killing perfectly good injectors seemed more likely: time to experiment. By this time, there were already a couple of manufacturers in the Duramax pre-filter business. Their sales pitch? They claimed that the fuel filter on the Duramax was nowhere near adequate enough to keep typical contamination from damaging the injectors. We had a supplier send us a kit.

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!

Three Words: Maintenance!

Just like grandpa said, water is a killer for any diesel fuel system. Now, with common-rail direct injection, preventing water contamination has become even more critical. So maintain that fuel filter and water separator! As mentioned before, GM recommends changing the fuel filter every 15,000 miles or 24,000 kilometers. (2004s and newer with the latest ECM and Instrument Panel software have a filter life monitor that calculates the fuel filter life based on total fuel flow.) And do not overlook the recommendation (and my Grandpa’s lectures) to drain the water separator at every oil change.

The need to replace the filter every 15,000 miles is well illustrated in the samples depicted on the previous pages Take a look at the inner filter media. As you can see, it is normally paper-colored. It starts to plug, from the bottom up, with black asphaltenes. The filter that was changed at 17,000 miles does not show very much unrestricted media left. Running this filter for much longer could potentially damage injectors. The filter that was changed at 10,000 miles shows a fair bit of life left. The filter that was changed when it was absolutely plugged (the engine would hardly run) shows almost no free filter media. In fact, I had to look for a section of the inner filter that did show a bit of color. Not surprisingly, the injectors on the truck that this filter came off of were damaged.

Duramax diesel fuel filter at 10,000 miles

Duramax diesel fuel filter at 17,000 miles Duramax diesel fuel filter at unknown miles

Samples of the outer and inner filter media taken from several filters. The outer media gets completely black, the inner shows it’s original color. (Click on any image for more information and to enlarge)

Take Care of Your Duramax Fuel Filter!

I trust it is obvious by now that any diesel owner needs to keep up proper maintenance on fuel filters. Even the much-improved fuel filters that GM provides today can be defeated if they are left on the truck until a problem develops from restriction. Changing the fuel filter is a much cheaper option that replacing all eight fuel injectors!

There is nothing wrong with having a good pre-filter system. It will provide an extra layer of defense against contamination and water. If you have any doubts about the quality of the fuel you are getting, I would definitely recommend a good water separator and pre-filter system. And it is worth mentioning again that if you run a fuel additive, use one that will help the water separator, not hinder it by emulsifying the water in your fuel!
Personally, I run just the AC Delco Duraguard fuel filter. I use reputable fuel suppliers, drain the water separator at every oil change, and change the fuel filter based on the fuel filter life monitor. (which usually works out to around 15,000 miles depending on idle time and fuel economy) My Duramax runs fine, as do most that have regular maintenance. That lesson that I learned from my grandpa hasn’t changed over the years. In fact, it has become even more critical as engines have gotten much more sophisticated. So take care of your fuel filter!

One More Thing

You might be asking yourself that if the fuel filter is so critical to injector life, why does GM have a special policy warranty on the LB7 Duramax injectors? Though the fuel filters have shown themselves to be the cause of the majority of injector failures, it is still true that some of the materials in the injector could stand improvement, more notably the well known ball-seat erosion issue (covered in the Winter 2007 issue of maxxTORQUE). However with good fuel quality, good filter maintenance and a pre-filter, some trucks are still running on the first generation injectors. For that matter, a few are still running without having a pre-filter. I believe that it is fairly safe to say that the improvements in injector materials were made largely to better handle the less-than-perfect realities of fuel quality and filtration.

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Comments (1)add comment

Mike Malone said:

Disturbed 3 sets, 216000 miles pre filtered from auxillary tank
What about an air dog?? I did not fully understand what you were talking about in regards to fuel treatments, What do you recommend????
December 15, 2013
Votes: +1

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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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