2010 Mercedes W212 E Class E350 CDI Sport, 2006 Mercedes W169 A Class A170.
OK.....so now its leaking coolant....both outside and into the exhaust!.....but its only a bit from the EGR steel gasket. I knew it was not great when I re-used it.....and then I got massive amounts of steam during a short trip!....just to make sure it was that vI put a smear of silicone on it and refitted it.....no steam.....for about 5 miles.....then its like a steam train again....a small amount of water makes a pretty spectacular amount of steam!!! Its not the best design with only a very thin amount of gasket metal between the coolant passage and the EGR passage....In the pic below the coolant passage is the small round hole bottom right.....about 2mm from the EGR valve! New gasket and O ring on the way! Not sure whether I should use some sort of sealant too....or which sort....as a belt and braces measure!!!
Turns out it standard.......not sure what it does yet but its connected to the fuel pump on the front of the engine and it can clearly be seen in this strip down video about 3 mins in (fuel pump cooling??)....so I take back my comment about the previous owners mod!!!