No pressure ,or no vacumm , have it your way. But it will run poor ,and even stop , with a bad fuel cap seal.. May be i said it wrong .But if no sound from it as you lift off the filler cap ,the cap will need replcing ..

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.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

What is the cause of all the air that comes out when you undo the cap?

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio