Best guess:
Breather hoses broken, loose or failing to seal..
This allows unregulated air to enter the system = P0173


Vehicle: 1999 Mercedes Benz E 430 Sedan
Part: Breather Hose
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Breather Hose
Partial load
crankcase ventilation
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Breather Hose
Partial load
crankcase ventilation
-----------------------------------------------------
Breather Hose
Partial load
crankcase ventilation
-----------------------------------------------------
Breather Hose
Full load
crankcase ventilation


http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1US0YZ0AN1UU0Z62DB&year=1999&make=MB&model=E-430-001&category=B&part=Breather+Hose