[C320-list] Yanmar Question

[mike hunter]

Question for the Yanmar gurus:
 
There is a wire tied threaded cap to the left of the injector pump on my 3GM30F that is loose and leaking a little fuel.  The wire is preventing the cap/nut from loosening further.  Before I tighten it I was hoping to get some information on the purpose of this cap and why is it tied off with wire?  Is it some type of pressure relief valve/vent?
 
Thank you!