The Black, White and Greys of the Duramax Fuel Filter

Volume 1 Issue 2 - Diesel Articles

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

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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