stock gas tank question

[archie307]

are stock gas tanks supposed to have pressure? I open gas cap and a bunch of pressure is released. I always thought carbed engines the gas tank was vented? It's getting to the point where you can hear the tank collapsing as you drive through a tank of gas. can anyone shed some light on this? oh this is on a 72 GMC. gas tank behind the seat. was planning on pulling the gas tank this weekend to clean it out and see if i see anything obviously wrong. oh and it doesn't fill up all the way. it fills up only halfway and i can;t get no more in. maybe i'm just being retarded. sorry.

[lowriden58]

only filling up halfway? how many gallons does the pump say when your done pumping?

[archie307]

from almost bone dry....about 13 or so gallons.....

[bob1967]

When I had a behind the seat tank in mine it did the same thing, I would take the cap off and it would blow off. If I remember correctly there is a vent hose on the fuel sending unit on 70 to 72 tanks, don't recall one on my 67 tank. I was tired of waiting for the tank to just explode , so I put one in the rear under the bed

[70inline6]

The fuel system on a 72 has a constant vacuum on the tank to draw fumes into the vapor canister/carburetor. The gas cap should allow air to be sucked into the tank, though not allow fumes out (like earlier caps). THe positive pressure bob referrs to above is usually due to a defective purge valve on the charcoal canister.

If you've changed your gas cap (so that it fits tighter or something), it's possible you've gotten one for another application. Earlier trucks have very little vacuum and it's only due to the fuel level dropping in the tank, not engine vacuum.

Try another cap and make sure the vacuum system for the tank is routed properly.

[lowriden58]

Originally Posted by archie307

from almost bone dry....about 13 or so gallons.....

this is interesting.. cuz i just read the other day that they are suppose to be around 20gallons. BUT i have never been able to get more than 13gallons in mine aswell

so im gonna see what others say on this subject

[archie307]

well i have changed my cap. when i went to the parts store they had two options. maybe i grabbed thw wrong one,...