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All fuel tanks have a vent valve to redirect vapours from the fuel. During a fillup the turbulent flow of the fuel causes quite a build-up of vapours that need

All fuel tanks have a vent valve to redirect vapours from the fuel. During a fillup the turbulent flow of the fuel causes quite a build-up of vapours that need to be redirected. Persistent fuel overfilling causes fuel to flow into the vent valve, and over time, it clogs. Once clogged, the vapour has no outlet and bubbles up through the main opening, causing the overflow you see on your Mustangs. It’s not the pump’s fault, although the problem is exacerbated when the nozzle does not fit fully into the tank. Car manufacturers always tell people to stop filling after the first click, but most people tend to overfill their tanks regardless. I’m guilty of this too. Source: Prior stint as a gas jockey and station manager

serf 6y ago
>All fuel tanks have a vent valve to redirect vapours from the fuel. During a fillup the turbulent flow of the fuel causes quite a build-up of vapours that need to be redirected. vented fuel tanks began in the 60s. Many older cars have absolutely no vent valve to speak of -- the vapor is dumped from the fuel cap, which acts as a vent; meaning the vent isn't present during fill-up on this style of design.
sokoloff 6y ago
The vent on the mid-60s Mustang is right back up the fill pipe. In car operation, the cap is vented. (It seems like this thread has told me why it’s not working.)