PeachParts Mercedes-Benz Forum - Mysterious 3/8" Barbed Vacuum Hose Fitting

1994 MB E320 Sedan
Hello All,
I've been searching around for a vacuum leak on my car for days now as it can be heard under slight acceleration and at idle, still have the fluctuating idle problem, and quite by accident, found this 3/8" barbed hose fitting underneath the cable guide and below the fuel rail. It is hidden unless the intake pipe, and cable guide cover are removed; My questions are: Does anyone know what this fitting is for?, Should it have a hose connected to it and be routed somewhere, if so, where?, Or, be blocked off with a blank cap? I've loaded some pictures which show the barbed fitting up close, then further and further away so you can see the fitting, and where it is on the side of the engine. Any suggestions would be greatly appreciated. Also, I had purchased an OVP relay recently (old one rattling) and just got around to installing it, and have noticed the ABS light stays illuminated a split second longer than usual after the bulb test at start-up; any ideas?
Thanks in advance