2008 lmm low rail pressure hard shifts
 

hey guys im helping my buddy on his 08 its tuned by duramax tuners has a 150hp tune i plugged my cts into it and it says at WOT the truck wants 21,000 psi and the actual FRP is only 2235psi and the truck shifts hard????? truck has brand new fuel filter and 138,000 miles i was maybe thinking cp3 is out

Does this truck have a Lift pump? and it's only at Wide open Throttle?

How is the truck at idle? smooth or is it lopey?

no lift pump and yes only WOT idle is smooth

Without being by the truck, its seems like it is not holding rail pressure, i would check the Cp3, it isn't holding pressure for the TCM to recognize when to shit

Spell Check** Shift.

double check the 2235 PSI. seems real low, likely won't run or will stall at that PSI. maybe a bad pressure sensor