He does have the 3.5, not the 3.0, so it is a "rod bender". But if it lasted this long without bending, its possible he is out of the woods...

When I got my 93 turbo, I had the same problem. No turbo. First, I replaced the OVP relay. That fixed it. Then, a year later, the turbo never spooled up again. This time, the computer was shutting it down (leaving wastegate open). This was because the EDS computer detected a vacuum leak in the EGR system. I decided to do what many on this forum did with the 300D's of this genre. Yank to stupid computer controlled system out and install a normal pressure regulated (spring) wastegate. That also disabled the stupid EGR. I assuming the 300SD might have the same "type" of system, but I have never seen one up close. Good luck. It could be something simple. When my turbo shutdown, It struggled to get to 60 on flat ground.

__________________
93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K