No..that is fuel rail pressure Test Port Fitting.

What we always check first when there is a fuel trim adaptation limit code
on EFI systems is the Fuel pressure REGULATOR vac port.

They are notorious for leaking by and that leak causes unmetered fuel to get sucked into the engine intake thru the vac hose , causing a rich condition that can be beyond ECU correction b/c it is raw gas that has by-passed all control systems [ un-metered fuel]
So, the test is to simply take the vac line off the REG. and look for eveidence of gas in that vac line..if ANY, replace the reg..it is leaking by...I also run the engine with the vac line OFF to see if any gas comes out the port..if YES , same deal..replace regulator..
That should have been done when you did the fuel pressure test b/c FP test is done with BOTH vac line ON and OFF. [ along with 30 min rest]
If no leak at reg , continue on with other possibles ..I mention this simple test only b/c of it fequency of failure rate on these and I have seen so many MAF and other sensors changed in vain b/c this FIRST test was not addressed.
Takes all of 15 secs., so I recommend getting it off the table ...
If it is bad , I also recommend my previous memorey Reset of ECU for Zero/Mean...after reg replacement.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2EI1B4MYE2ER0X76YW&year=1994&make=MB&model=E-420-001&category=C&part=Fuel+Pressure+Regulator

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A Dalton