Have you cleaned and checked,your ALDA lines? thats a big step in restoring proper boost levels. If not, there are alots of threads in the forum you can access via Google questions. Just type in Mercedes ALDA and you will find a ton of stuff about it. or you can go to (y@utub#) and find stuff along with videos.
:::::how to clean ALDA boost lines.
On your engine, the nipple the feeds boost pressure the to ALDA is on the top drivers side of the intake manifold, its a little brass fitting that has a short rubber hose cominig off of it then a hard plastic line going to the fender to a little switch over protection valve, then out of the valve and onto the alda which is the (what looks like a beefy fuel pressure regulator).
If that valve is not working or if that little nipple on the intake is clogged, you will get hardly any if no boost.
you can also reroute the lines so the (possible bad valve) is taken out of the equation.
just plumb the little intake line directly to the ALDA

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Re: Alda lines

Quick answer yes.

Long answer - when I did the glow plugs I cleaned the crossover and intake. They were sludged to the point of being nearly useless. You could eat out of them when I finished. I replaced the IM, SO and alda lines. I can put boost on the line that exits the manifold and the switchover starts venting to atmosphere at 14/15 psi. A gage at the alda reads to 14 or so psi when adding boost at the IM line. I believe this is working correctly.

I've measured boost at the IM and the Alda. They are exactly the same. 2-3psi.

Thank you.

ps - Alda seems to work - 1/2 turn ccw richened the mixture
pps - EGR screwed closed - should in theory have no more gunking