2004 Columbia w/Mercedes engine loses power

Hi!
I have a pos 2004 Columbia with a Mercedes MBE engine. Okay so have another one with the EXACT same engine, same gearing, etc, both pre EGR, and is a powerhouse (considering) this one though, can’t get up and going and won’t pull or hardly move until you wrap it up past 1500rpm. The other one however, just put your foot in it and she starts building NO problem; pulling and hauling butt!
Replaced the Turbo, fixed air to air leaks! What else could it be??? We are stumped!! HELP!!