ISX sudden boost psi change

[Carter]

I’m currently driving a 2015 cascadia with an isx15 cpl 4583 450hp 1650 torque. I normally accelerate through the gears (10 speed) with a light foot and hold the boost at 10psi. Occasionally on flat ground I need 12-15 psi boost to accelerate. Then in 9th gear at about 45 mph and about 1300 rpm there is a sudden increase in power and my boost drops to 8 psi and the truck preforms much better. Seems random but happens if I stop and idle for say 10 minutes but not all the time.
Truck is completely stock with 465,000 miles. Not sure of the gear ratio but it turns 1250 rpm @ 60 mph. It’s got me puzzled.

[Volvo8873]

1250rpm at 60mph is probably 3.25 gear ratio which is horrible gearing that will lug the liners right out of the engine! You should immediately regear to 3.90 or 4.11 if you expect to get a million miles out of the engine.

[Carter]

(09-21-2022 )Volvo8873 Wrote:  1250rpm at 60mph is probably 3.25 gear ratio which is horrible gearing that will lug the liners right out of the engine! You should immediately regear to 3.90 or 4.11 if you expect to get a million miles out of the engine.

It’s a company truck. I have to do the best with what I’m given.

[Bengy88]

If you only ever see 10-15 psi of boost you must be pulling empty plastic trailers down hill.
Worse thing about company or fleet trucks are their specs. It’s hard to explain to some people that a Diesel engine is made to work….. so they need to work. It’s like building a race car and putting around town.

[Carter]

80,000 gross pulling hopper bottoms. Majority is very flat ground biggest obstacle is wind. On rolling hills it’s allot harder to do. Not that I never ever get the boost up there but I look at it as 10 lbs is perfect, 15 lbs is redline and 20 is bouncing off the rev limiter.

[Bengy88]

Ok well you need to do what you gotta do to keep those rpm’s up whenever you can. This is why I hate when my customers buy Ryder and Penske trucks. They are mostly all specked with high gears, overdrive transmissions, limited to 60 mph, have progressive shift, and lugged to death. I worked for Ryder a few years and the specs I seen trucks have were crazy.
If you want you can send me the last 6 of your vin and I’ll screenshot the specs of your truck for you from freightliner.

[Carter]

It’s definitely not how I would spec the truck, but like I said I gotta do the best I can with what I’m given. I can get some decent fuel mileage out of it considering I’m heavy and do short hauls. I’m just wondering if that sudden change in boost pressure is something in the programming and although annoying is normal or if there’s something wrong. The truck seems to run just fine otherwise.

[Rawze]

If you cannot make full rpm (1500 - 1900+) then it is not likely you will get higher boost values because the truck is completely unable to make full HP below 1500 rpm.



also, a non-demanded truck is going to have all 13+ of its engine operating modes become active as needed to either keep the cans under the truck temp regulated, or other tasks. The boost, power and everything else is effected by this depending on what emissions, thermal management, or other mode is needed and when.

[Carter]

It probably is a particular mode it’s in at the time. I can get full rpm and boost if I want. Maybe I’m driving incorrectly. I just give enough throttle to get 10 lbs of boost let the rpm’s climb then grab the next gear and do it over again. In top gear I do the same and if I can hold 62mph at 10 lbs that’s the speed in run if I can only hold 54 mph at 10 lbs then that’s the speed I run. On flat ground at least.