Hi. Hope this isnt a silly question, but can someone please advise me what this is/ what it does. - its a hard pipe with a threaded connection on that sits just above the passenger side rear of the valve cover on my 2003 f150 (triton 5.4 v8). I just lost the cover that attaches to that thread - should I expect any issues until I can get a replacement? the two photos show the thread itself and the pipe. Thanks a lot.

 

Hard to make out in the picture but I believe it is fuel pressure test port on injector rail.
As for the cap tire valve cap same size.

 

Fuel rail test port x2

 

Thanks a lot for the replies.