Fuel Tank Vacuum on my '94 740iL

[Jazzman]

Why is it that whenever I fill up and remove the gas cap I always get vacuum in the gas tank. Is it normal? Does it affect fuel delivery to the injectors? Just curious if that's the reason for my poor idle quality and gas mileage.

[shogun]

that is a well known problem on the E38, I read about some cases where the gas tank even collapsed.
Check the tank de-airation system.
www.realoem.com

poor idle is also an effect of this.