Please help! ISX15 poor fuel mileage

[Nihalu]

Hello Folks
After checking several YouTube videos on Rawze's Channel and inspecting the truck accordingly I ended up in buying a 2016 Peterbilt 389 with a Cummins ISX 15 engine in it ESN 79854924. Its a 550 HP/2050 TORQUE WITH 3.73 GEARS ON 11R22.5.ENGINE HAS BEEN OVERHAULED AT KENWORTH in my area 138K KMS ago with new rebuild kit and new cyl head. Current odo is 908000 km.Everything is just fine with it except fuel mileage. Just to mention it used to run in mountains with 140k lbs gross. Now I run vans/reefers with 80k gross. Max I got out of it is 5.8 MPG. All stock after treatment.Recently changed IMAP, DELTA P AND other recommended sensors. Dpf doc all cleaned but no mileage improvement. Please help me if you have any suggestions. Call/text 7788358132.
Any help from legends around here will be really appreciated.

Thank you so much

[Coronado126]

Let's start with the basics.

Has is always been 5MPG or did it suddenly get worse? Are you seeing any smoke on start up, shifting, or when you pin the throttle?

Is that mileage from the dash, or by calculating it by hand with gallons used vs miles driven?

Have you checked/changed the air filters and fuel filters? Are you having any performance issues?

Do you have Cummins InSite?

[hhow55]

Go to quickserve.cummins.com
Under symptom search you will find information to help yourself.

[Mattman]

What is your driving style?
How much idle time do you have?

[SquareOne]

Also, have you pressure tested CAC and boots to check for leaks? Exhaust as well? How high is the turbo boosting at it's highest point? How much while cruising or climbing hills?

[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.

[Explosia]

It’s a square nose Pete, he should’ve expected this when he bought the truck. If he takes Jim’s advice, he can certainly improve, but he ain’t getting 7-8 mpg running 80k on the mountain grades of Canada.

[Rawze]

(11-19-2020 )Nihalu Wrote:  Hello Folks
After checking several YouTube videos on Rawze's Channel and inspecting the truck accordingly I ended up in buying a 2016 Peterbilt 389 with a Cummins ISX 15 engine in it ESN 79854924. Its a 550 HP/2050 TORQUE WITH 3.73 GEARS ON 11R22.5.ENGINE HAS BEEN OVERHAULED AT KENWORTH in my area 138K KMS ago with new rebuild kit and new cyl head. Current odo is 908000 km.Everything is just fine with it except fuel mileage. Just to mention it used to run in mountains with 140k lbs gross. Now I run vans/reefers with 80k gross. Max I got out of it is 5.8 MPG. All stock after treatment.Recently changed IMAP, DELTA P AND other recommended sensors. Dpf doc all cleaned but no mileage improvement. Please help me if you have any suggestions. Call/text 7788358132.
Any help from legends around here will be really appreciated.

Thank you so much

so you buy a fuel pig of a truck with a front end that looks like a 4x8 sheet of ply-wood facing the wind .. and then you go and haul low-rent freight with it (dry-box/reefer) -- don;t comlain about fuel mileage in that pig of a truck made for a museum.. they should be outlawed in that market. - Sell it and buy something competitive (prostar or cascadia w. ISX and spec'd right for the job) in profitability and start over .. that is my advice.

Best you can hope for is high 6's in that pig... it ain't no truck for the low-rent market your in... and you buying it, either you knew this already or you did not do your homework at all towards a solid business model that actually involved profit over that antique-roadshow looks.

5.8 sounds about right if your pushing that pig down the roads at 70+ like a complete fool. - Start driving by your boost gauge, keeping it under 20 at all times, and drop the speed to about 60 of you even want to stand a chance with it in the market your in.. refuse any load that required more than 600 miles in a single driving shift so that you can keep the speeds down and the profatability somewhat in check.

[Nihalu]

Thank you all for all your advice and helpful tips
As you guys pointed out, I always drive at 59 max and try to keep my boost under 20 at all times.
Its been like this from the beginning. I m not hoping to get a 7 mpg out of it but I want it to at least do 6.3-6.5 mpg.
Thanks again for all your help

Drive safe