See if there is any evidence of petro in fuel rail Regulator vac line.
If YES, then the Regulator is leaking unmetered fuel into the engine intake.
Common fail part....

http://catalog.peachparts.com/ShopByVehicle.epc?q=1996-Mercedes--benz-S3--0--Swb-Fuel--Injection&yearid=1996%40%401996&makeid=63%40%40MERCEDES%2DBENZ%40%4063&modelid=6430%3AMBC%7C1514%3AED%7C214%3ASM%7C853%40%40S320+SWB&catid=240621%40%40Fuel+Injection&subcatid=240645@@Fuel+Pressure+Regulator&mode=PA

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A Dalton