I've had various boost problems over the years (which makes the 2.5T a joyless ^%$#%&$# dog). Not being a confidant DIYer and having no patience with power loss, I had my indy dial in the solution (s).

One fix was to UNPLUG and DISABLE the boost over-protection valve (sensor), which fixed more than a year of frustrating on-and-off boost (while some might say that's as foolhardy as going to a cathouse without a raincoat, I'm confidant -- and more importantly, so is my mechanic -- that MB overdesigned its turbo overprotection system and that there is little danger in popping the turbo under max boost).

When the turbo boost problem resurfaced recently, it turned out the problem was a minute, hard-to-detect vac leak.

My 2.5T now pulls with all 121 horses and instant get-up-and-go. In short: it's an utter JOY to drive.

Good luck with your turbo issues; once they're sorted you'll treasure your w124.

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'91 300D 2.5 Turbo 330K
'00 VW TDI Golf, 190K
'67 BMW R50/2
'73 Norton Commando Interstate