Please help! ISX15 poor fuel mileage

[JimT]

First thing that comes to mind when talking about fuel mileage (isn't it weird talking about fuel "mileage" when you measure distance in kilometres? anyway...) is the driver and driving habits. There may or may not be mechanical issues, but driving habits tend to play the largest role.

How fast do you drive? 58-62 mph (not sure the kph equivalent) is usually the sweet spot.

What rpm range? According to Rawze 1500-1800 is the best for horsepower and engine longevity. On climbs it's usually equal, or slightly more, fuel efficient to run higher rpm/lower boost as opposed to higher boost at lower rpm.

Do you have a boost gauge? Do you try to limit your boost pressure to 20psi or less under engine loads and 10psi or less when cruising?

Idling not only wastes fuel but also leads to more frequent regens due to plugged up filters.

Cruise control is nice and helps keep your foot from cramping up on long stretches, but for anything other than flat ground it's not ideal. Any hills or mountains are best tackled with a light foot and an eye on the boost gauge. Grab your speed on the downhill, either in gear or out of gear, then find a boost target, like 20psi, and hold steady at that boost level, downshifting as needed. 15psi fully loaded on a good grade is totally doable if your willing to go slow, though as you drop below 45 mph the fuel savings diminish and below 35 mph it hard to notice any additional gains from slower travel/lower boost.

Even when using cruise control it pays to do some testing. I've found that setting the cruise at slight differences in RPM can actually yield big gains. For instance I can set the cruise to 62 mph anywhere in the range of 1540-1560 rpm in 12th gear, however 1560rpm yields up to 1 mpg better results. It's a little different for each gear, but try playing around at slightly different rpms and see what your engine likes best.

Aerodynamics, terrain, temperature and gross weight all factor into fuel mileage as well, but they are largely static. That is, it's not something you can easily change.

I rely heavily on my boost gauge and the fuel economy reading on the dashboard. Long term analysis has shown it to be fairly accurate on my truck. But, no matter how accurate or inaccurate it may be, the better the number being reported, the better the actual values will be.

When I pay attention and take my time I can usually get low to mid 8's, maybe high 7's heavy through the mountains of PA. When I speed up or spend a lot of time with cruise control I tend to drop low to mid 7's. I used to be down in the mid to low 6's before learning about and implementing all the above, along with a few months of testing to find the sweet spots for my truck.