Originally Posted by nate_fisher
Its the fuel pressure regulator its a common problem with the newer sti's my friend had the same problem with his 2012 with a 2.5xtr. He installed a 2007 sti fpr kit and agency power parallel rails and lines and no more stumble between 2500-3000 rpm. To bad he fried his rings befor he got the chance to correct the problem. He put in new cp pistons and still had the stumble until he changed out the regulator and bam no more issues he is making 450whp on e85 now.
Alot of good tuners know this already and will use aftermarket or the older sti fuel regulators.
As far as upgrading your uppipe and turbo inlet has nothing to do with running lean but only leaving some HP on the table. I think u need a new tuner that knows what he's doing as he shouldn't have left u in this state with proper knowledge on how to fix your lean condition