Just went from a 2003 379 Pete with a 550 Kitty, 18 spd and 3:36 gears. It would pull like crazy through every gear up to 2000-2100 rpm (if you wanted to).
Now I'm in 2006 379 Pete with a 500 ISX Cummins 13 spd and 3:55 gears with 64,000 miles on it. In any gear it feels like it falls on it's face at about 1600 rpm. If I keep my foot on the floor it will slowly work up to about 1800 but it quits pulling at 1600, the boost falls off and everything. For some reason 4th gear is totally worthless, I'm lucky to see 20 psi for boost. Usually it will run at about 31 when it's pulling a hill. 5th gear starts bucking at about 1500 rpm before it stops pulling. 6th and 7th are so so, and 8th through 13th are usually about normal. ( I don't really know what normal is on this engine though) The thing that really gets me is every now and then it will let loose and pull like a freight Train through one or two gears clear to 2100 rpm, then it falls on its face again.
Peterbilt has looked at it and couldn't find anything. Cummins put it on there dyno and can't find anything. It usually sits for about a day before they can check it out and then it won't act up. I think it's gotta be good and hot before it acts up.
Any Ideas????

I think that may be normal.

In the 05 volvo with the isx400 cummings, same motor different boost and fuel, the thing pulled great to 1500 and ok 1500-1650 then pretty much fell off over 1650.

Redline on the 400 was 1900 and it cut out there, some gears you could over rev, but for the most part if you messed up and forgot to shift from 1400-1600 you sometimes got locked into gear and had to use the clutch to drop out.


K

In truck driver training the 2000 Western Star we drove had a Caterpiller in it, its designed to be run up to 2,100 rpms but the Cummins ISX engines are designed to give best power in the 1,200-1,600 rpm range for better economy and such...

with the FLD's we were first trained on some had the N-14 Cummins, you never ran those above 1,700 rpms

With that Western Star, it had a 13 speed and when starting off grossed out at 76,000 they had us run it up to about 1,500 till we got the higher gears then took it up to 2,100.

With the Cummins they has us shift the first 5 gears (10spd) all under 1,500 rpms then in the upper gears shift at around 1,600.

Progressive shifting can make a difference

Cummins shouldn't be acting up like that, or cutting out in some gears. Most likely your EGR valve is getting ready to go out. Probably the reason why Cummins or anyone can't find it is because they would have to run the EGR/VGT test while the engine is at operating temp. I've ran into this a few times and that what it ended up being.

If the geardown protection feature is set on the Cummins engine, the engine will be limited to 1500-1600 for shifting in the lower gears when lightly loaded or empty but will allow the engine to run all the way to the governed 2000 rpm when shifting gears under heavy loads when it senses the need for power, like when climbing hills.

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Wow, good info so far. Gives me a couple more tricks to try to figure out what's going on by the seat of my pants. Although, any more with all the computerized crap on them these days "seat of the pants" isn't worth much any more.

PackRatTDI,
Tell me more about the geardown protection. Can it be turned off? If I'm running around in town and going through the gears up a steep enough hill it seems to run like it should but it's still a little inconsistent.

i drive an international with probaly the same isx you have 10 speed tranny which is beside the point and i will tell you the best horsepower with the crappy motor is bewteen 900-1400 rpm anything more and it gives up anything less chugs the motor the funny thing i see in this is when they rated the horsepower of the motor it was at 2100 rpm which mine will not get up to in any gear unless i miss one and wind the motor out

It's a programmable feature in the ECM, like roadspeed governing or idle shutdown, so it can be shut down through a Cummins computer datalink.

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I'm driving a KW900 with an 475 ISX and poor poor performance! Maybe I'm use to a Kitty but this ISX sucks. I've been haven problems with the turbo and the EGR. We were have those problems to going from 4th to 5th were the boost would seem to stick at 20psi then it would bounce up to 40psi. One day I was driving down the highway starting out with a full load on just pulling a tri axle tanker and it was in 3 hi going to 4 lo all of a sudden my boost was climbing to 30 and up and then i lost my power no boost etc i looked in my mirrors and the isx was just poorn out the smoke Talkn about no power it sucked. Off the truck goes to KWhopper to get a new EGR. Then we have power again. Couple months later turbo problems and had to get a new turbo, check engine light comes on a week later with the new turbo and i lose 10psi boost goes off and i gain my 10psi back holy crap what a pain in the butt! Cummins I wanna Pete with a ##### CAT