just recently my turbo GSR sputter when my wastegate open at 12psi. during boost it act normal and accelaration is normal but once I hit full boost 12psi the car sputter (puhhh puhh puhh) and the rpm climp up slower than before. The sound when my car does this sound like when i used my anti lags 2-step launcher. I'm running Neptune on my car for 3months now, everything is with the car is fine (compresion,sparkplugs,ignition,etc.). I'm thinking either the wastegate is mess up or my turbo.

 

hows the a/f? breakup problems tend to be that

 

and have you tried any other boost settings?