I haven't put a ton of miles on my car since owning it (between it breaking and doing other build changes), but multiple times in the less than 1k miles I have put on it I have had trouble fueling it up AND there have been a few instances where I get some backpressure in the tank that can cause fuel to spray out under the car directly below the filler neck after the car has been running a while. I thought the external pump was getting hot and vapor locking but I felt of it and the lines on both sides of it the other day and it was as cool as could be. I just went to add fuel to the car using a race jug (after the car has been sitting and is cold), and no matter how I position the nozzle or jug or funnel, I can't fill it faster than a trickle. The car is a 2018 model so I know it should have the updated filler neck with the new anti-siphon setup in it. The car only has 8800 miles on it, but it has been on e85 most of its life. The only thing that makes sense is that the evap canister is clogged and causing a ton of backpressure in the tank. We are about to drop the tank and do a fore fuel system install so it won't be too hard to check everything out, but I was curious if anyone else has had this extreme of an issue with the fuel system.

 

You may have a stuck closed EVAP vent valve…the vent valve is normally open…if closed you may have difficulty filling up !!

 

Yeah I had the same issue on my 15, replaced the valve first and still had an issue. It ended up being the canister. I replaced the purge valve, evap solenoid and canister.
Not a bad job it all sits in the rear passenger wheel well. I've got some pictures Ill have to dig up. Think I spent Maybe $350 400 bucks for all the parts I cant remember.

-Rich

 

Found the issue and the cause. One of the shops that had their hands in the car before I got it did some sort of auxiliary fuel return line that plumbed into the top of the filler neck. That in turn caused the evap canister to fill with fuel. When we put the new fuel system in we did away with that return port and cleaned the evap canister out and its good as gold now.

 

Have not had that problem on my C7 but had it on a previous vehicle I owned, had to replace the charcoal cannister. My mechanic said that the common cause for this is over filling the fuel tank when you are getting gas. He told me that when the pump kicks off, pull the nozzle out, do not squeeze another nickels worth in to tank, said that is what causes the problem.