Turbo underboost on Mercedes OM651 (2.1 CDI) and OM642 (3.0 V6 CDI) — almost always a sticking VGT actuator, split charge pipe or worn turbo vanes, not a dead turbo.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.