I have ran this fuel cap for over a year.

When fueling up in Winter time, I pull off the cap, an ounce of water comes out of it. Yesterday, we have Heavy Rain, I jump in to start up, and the truck is running LIKE Crap. Hesitating, stumbling, and I shut it down. Pull off the fuel cap, over an ounce of water in it. I open the hood, switch the drain valve for a few seconds, and close it. Start the truck, smooth idle. This cap is not venting, to much moisture build up, not allowed to vent.
Stock fuel cap back on, no water in it, working properly!
FYI