Yes that green cap that you are talking about is your emissions evap service port (or something like that) There are 2 vacuum lines that run around that area. When I first got my truck i had an evap leak that would come and go too, and it wasn't the cap. I think I cut and fixed the lines about 3-4 times, and the hoses kept splitting so i finally decided to replace both of them. I think the job took 5 minutes and cost about 8 bucks. Best of luck.