I have the same exact problem cart is fine in first and second then in third it becomes a dog till it gets into over drive and it moves past 35-40mph

I try not to push past the 2500rpm range and I cant hear the turbo spool till about 3000 rpm if I push it further

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2003 GX470
1998 E300 3.0TD
2002 Dodge 2500 4x4 cummins
1998 Dodge 3500 5 speed Cummins