Flaps? What flaps?
AO Silver Member
Join Date: Oct 2014
Location: United Kingdom
Underboost would either be:
1. VNT veins stuck in the low-boost position because of sooty deposits in the turbo
2. Fault in the VNT Vacuum actuator system, e.g. there's not enough vacuum to move the vanes into the high-boost position
Check the small vac lines that run from the turbo actuator back to the boost solenoid, check the solenoid isn't brittle/cracked (there's also a small filter hidden away in this solenoid that you can inspect), and then check the vac lines from the solenoid back to the vac reservoir and pump... Splits aren't uncommon in the vac hoses, and would prevent you from getting into high-boost position...
It's also worth removing the vac line from the actuator on the turbo itself and seeing if you can move the actuator arm.. Should move by about 10-15mm in/out to run between low/high boost positions... If that's seized solid then your turbo needs some loving!
Any smoke from the exhaust pipe at all? That's not uncommon with an underboost problem...