I bought a smoke tester* and hooked it up to the purge valve line running to the back of the car. After figuring out that I had to cap the air intake/filter for the evap system (#7 at link), I was able to find the leak. It's the small pipe attached to the main tank filler pipe (see small pipe in part #13 below). Looks like it has been rubbing on the frame for the past 160k miles, slowly wearing a good sized hole into a segment right up where it is basically inaccessible (can either see it with a mirror, OR feel it with my fingers -- probably halfway between where the #13 label is pointing and where it attaches to the tank in the diagram).
My question is: given that I don't intend to drop the tank on this car or actually properly replace this pipe, but that I do hope to get a few more years out of it -- what sort of epoxy/rtv/other adhesive would you all recommend using to patch the hole? It is rubbing up against the frame, so obviously I'd have to put a paper towel or similar over the adhesive to avoid gluing it to the frame. I've seen JB Weld's TankWeld -- is that a good option? There isn't any room to cut the pipe and splice in a hose. I'm sure the AWD P2 owners out there are well aware of the tight quarters around the fuel tank.
Any and all advice is greatly appreciated!
* I was surprised by the quality of this tool given the price. I'd recommend it for anyone looking for a home smoke tester. This is the heavier-duty version -- there's a $90 version with no built in pump and a little cheaper materials if you want to save $40. I don't know the manufacturer, and I'm not getting paid to advertise.
https://www.amazon.com/dp/B0B2JV8YDP
