Overboost and Underboost Subaru common causes:
1.) Solenoid lazy or not working – Cure: Clean use carb cleaner or replace the solenoid.
2.) Restrictor Pill not fitted / size incorrect – Cure: Ensure the restrictor pill is fitted (2 port) if so on 2 port reduce the restrictor size. On the 3 port increase the port size to increase boost to reach normal safe max boost level.
3.) Weak or broken actuator spring – Cure: Replace the actuator. This can be tested for by connecting the turbo outlet straight to the actuator and seeing what max boost level you get this should be about 0.5 BAR.
4.) Leaking exhaust manifold and/or up pipe. The production of boost via the turbo is very dependent on exhaust gas flow over the turbine. So if there are any leaks in the exhaust pre-turbocharger your spool and peak boost will be affected. Obviously the larger the exhaust leak the worse the effect on the turbo output will be.