Originally Posted by Diseasel300

Again, no. The only reason the cap is sealed in the first place is for emissions (preventing vapor from escaping) and to minimize moisture ingress due to temperature fluctuations.

The tank has a vent to prevent building pressure and has a valve to prevent vacuum. You can remove the cap and drive around all day and have no impact on how the car runs or drives. On a newer car, you can set a check engine light if the purge valve checks for vacuum at the cap (most cars run very slight vacuum to prevent fumes escaping), but this has absolutely no effect on how the engine runs.

All fuel pressure is created by the fuel pump. It doesn't push air in the tank to do this. Not sure where you're getting your information, but I'd check the reliability of your source.