[C5GTO]

I've been experiencing something different and am curious to understand what might be causing it. The gas tank in my Cobra is becoming pressurized when I drive the car. It seems to run fine but I do need to loosen the gas cap very slowly to let the pressure escape or it can burp raw gas out the cap.

The car has a 5.0L Ford with the early 90's style Ford EFI, so it's a return style system. It has the LeMans style gas cap mounted in the passenger side fender top and I use a OEM screw-in gas cap under the flip cap. There's a couple of things that might be contributing but it's not clear to me they are. The car was stored away for about 3 years and not driven. Prior to putting the car in storage, I regularly drove the car and there were no indications that the fuel tank was being pressurized. I also recently moved from CA to AZ, so prior the car was getting CA formulation fuel, and now it's getting AZ gas. I guess the other change is fuel formulations may have changed over the last few years and especially with the current fuel situation.

The first symptom of the issue is that gas was bubbling up out of the LeMans cap while I was driving the car to the extent that it would run down the fender. It would start bubbling out after about 15 minutes of driving. The gas would bubble out until the cap was loosened and there would be a big whoosh of air (and gas) until the pressure in the tank dissipated. It would be good for the next 20 minutes or so of driving and then start bubbling gas out again. On closer inspection, the gas cap O-ring was not sealing and thus the gas could be pushed by it. I modified the cap so it could screw down further, completely seal and this stopped the gas bubbling out.

My next stop at the gas station told me I had effectively sealed the system. When I loosened the screw-in cap, a bunch of pressurized air was released along with some liquid fuel. I have learned to loosen the cap very slowly in order to slowly bleed off the air to avoid having liquid fuel burp out. So the situation seems under control but it still worries me some.

Has anyone else experienced something like this? I'm guessing this is "normal" for an EFI car where the fuel is being constantly recirculated and likely being warmed during this process. But I thought it worth while putting the question out there in case there's an underlying issue that I should address.