Does anyone know what this fitting is????

[acabtp]

fuel pressure test port (for FICHT testing). plug it.

[bulletbob]

Originally Posted by acabtp

fuel pressure test port (for FICHT testing). plug it.

Correct... It really should be there, not leaking, but if all else is ok, you can certainly run with it plugged for a while with no problems.. Typically that fitting is used when there is a running problem and you[or a technician] would need it to put a fuel pressure gauge on and check the fuel pressure against specification.
On automotive systems, it is also used to run fuel injector cleaning products through.. not sure if that is done the same way in marine applications.
I have also used that port to check for water in the fuel at times over the years, and it also works well for that. You can access the fuel without taking everything apart. For now, plug it, when time allows, get it fixed right... bob