Check that there's a clear line from the intake manifold to the square thing on top of the injection pump. Depending on the vintage of 2.5, the line from the manifold can go through solenoid then split to the square thing and a pressure sensor, or there won't be a solenoid. You should be able to blow air in one end and out the other. Run a pin or wire through the manifold fitting to make sure it's not gummed up. You can remove the fitting on the square thing to clean it but don't poke into the square thing. Don't apply a vacuum to the square thing. Square thing is the technical term for what most folks call ALDA. ALDA is the search word here but it's debated on a dozen threads daily so it's hard to find definitive guidance.

Check that the vacuum line to the wastegate actuator isn't leaking. Also check that EGR wasn't disabled using a blocking plate or plug in the vacuum line.

Depending on what you find, this might be the simpler path - http://www.peachparts.com/shopforum/diesel-discussion/212097-wastegate-actuator-swap-om602-962-a.html

The downside of the modification is you pretty much have to do it yourself. A self-respecting shop shouldn't tamper with the emissions system.

Sixto
87 300D