Don't tighten it!! Whats missing is the "end cap" for the injector return lines. A small piece of return line with a plug in it. You're loosing an enormous amount of fuel through there you know, I bet easily half a tank in 20-30 mins of driving! 
See the white lines coming off the rest of the injectors? The end injector has a little piece with a plug in it. Yours popped off, and needs to be replaced. Super easy fix, they just slip on over that barb.
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