![]() It’s impressive how modern manufacturing can mass-produce such precise and accurate parts, but those tight tolerances and high pressures create an injector which is very sensitive to contamination from water and dirt. In order to operate at pressures approaching 27,000 psi, the pieces inside common-rail injectors need to be machined within a couple microns of each other for proper tolerance. Manufacturers found early on if you can inject diesel fuel into an engine at a higher pressure through a small orifice, you can greatly improve how quickly and efficiently the fuel will burn, and also reduce the output of harmful pollutants. While there are no inherent strength differences between the two piston designs, you have to be especially careful to monitor EGT on the early design engines. On the later trucks, the narrower spray pattern and piston design gives a little more margin of safety and allows for increased injection duration, since the piston will be in the “sweet spot” for more degrees of crankshaft rotation. On the earlier engine, the wider-angle spray pattern means the piston has to be closer to top dead center when the fuel is introduced, so when tuning there is a limit to how much injection timing and duration you can safely run. To ensure your engine lives a long, healthy life, its imperative to keep your injectors in tip-top shape. The third injection event is said to have a slight negative impact on fuel mileage, however with aftermarket tuning, the loss in mileage can be restored. In addition, the later 5.9 Cummins also featured a third, post-injection fuel event after the main spray, and the main purpose is to reduce harmful emissions. The later 04.5 to 07 Cummins 5.9s featured a five-hole nozzle, a narrower spray pattern, and to match that pattern, a different shaped piston bowl needed to be used. The pilot injection event is useful since it reduces noise and helps the main combustion event take place, and interestingly enough the addition of pilot injection is why common-rails are so much quieter than the mechanical engines which have only one injection event. ![]() The early 5.9 CR featured an 8-hole injector nozzle with a wide angle spray pattern, and there were two injection events per stroke: pilot (which injects a small amount of fuel to pre-heat the combustion chamber before ignition) and the main injection event, which is where the engine’s power comes from. ![]() While they share most of the same architecture, there are a few key differences which make certain parts not interchangeable, and most of those had to do with fuel delivery. The 5.9 common-rail is divided into two separate groupings: the earlier 2003-04 models and the later 2004.5 to 2007. As part of our “What Breaks When” series, this week we’re taking a closer look at the 2003 to 2007 5.9-liter Cummins. While the 5.9 Cummins is a very robust engine, it does have a few weaknesses that can leave the truck stranded, but nothing that will lead to a catastrophic failure or meltdown.
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