Fuel Tank Vacuum After Driving?

I've noticed that my fuel tank seems to pull quite a vacuum after driving. When I open up the filler there is a big suction pulling the cap down and an audible "whoosh" once I break the seal.
I'm thinking that this isn't right and maybe my venting system is plugged somewhere.
Can anyone advise?

Thanks
Kent

This is not good as it can make the car more prone to vapor lock. Is this on your 73? The 73's have a charcoal canister venting system(US cars) and if it is not functioning properly can cause this problem. Someone here posted a schematic and you need to check all those little lines. They may be collapsing or crushed.

There is an evaporation tank in the left fender that allows air back into the tank. Mine and probably most others crack overtime leading to a strong fuel smell and worse. Me and probably many others then block off this line which solves the problem but causes the vacuum issue you have. Check this line and I bet it is closed off. I solved the problem permanently by putting a one way valve on the line so air can go into the tank, as fuel is burned, but cannot go out.

Chris
73 911 E

Aha! Thanks for the advice Chris & David. I'll have to investigate the lines for the vapor system. I'm pretty sure my evap tank is intact, but there is surely a block somewhere. It seemed counter intuitive that the tank should pull a vacuum.

Does anyone have a line (pun intended) on replacement clear lines for the vent system? They look like plain old Tygon to me.

-Kent

The correct amber lines are available from McMaster -Carr Kent. I think I and others have posted part numbers already. If a search doesn't find them for you I'll see if I still have the numbers recorded here somewhere.

Thanks for the tip Ed. Per your suggestion, I've found the threads on the site. Can you advise if I should go for the clear tubing like some have used or, the yellow fuel resistant stuff on McMaster -Carr?

Thanks again.

Kent

I prefer the amber fuel vapor rated hose. One, I like the correct look, and second, I like the fact it is rated for fuel vapor, unlike the general clear poly hose from the typical Ace Hdw. source. Not that the clear poly doesn't work,,,,,I used to use it before I found the better stuff!