New gas tank that thumps and bangs

[RottenDog]

I recently had a custom 40 gallon gas tank built for my J10. It is made out of 3/16 steel and baffled. It staddles under the bed to under the driver's side of the cab.

The problem I am having is there are random expansion and contracting bangs/thumps being emitted from the tank. I know it has to do with the change in tempature, etc., but it is annoying and kind of makes me nervous. When I remove the gas cap, the vacuum is so great, you would get a severe hickey if you stood too close.

There is a vent hose that runs from the tank and connects to the hose for the filler neck. I changed to an 80s style bed and filler neck prior to the new gas tank. I have a stock 80s style gas cap. At the moment, I don't know if it is vented or not. Are there vented and non-vented gas caps for this vehicle?

My question is: Would it be safe to tap into the vent hose just before it connects to the filler hose with a vent valve of some sort to relieve some pressure or can the expansion just be managed with a different or vented type gas cap?

Can a small hole be drilled into the gas cap somehow for venting?

Or is there another safe and sane way to deal with this situation?

[joe]

If the tank vent goes back to the fill pipe you need a vented gas cap. Mid-late 70's Wag's were vented to atmosphere so just get a cap for that era.

[Rande]

I think the later trucks were vented to the charcoal canister. Can anyone verify or disprove? If its not presently vented, it seems the fuel pump would be adversly affected.

[joe]

Yeah Randy the later rigs vent hoses went to the charcoal can as did earlier rigs. Seems mid-late 70's or so they went back to vented to atmosphere for a few years for some reason.

[Rande]

There! You see! I'm not as dumb as I look!

[SBJeep]

I have the expansion/contraction bang in my 33 gallon Aerotanks gas tank. It has always done it. I use a gas cap with a big hole drilled in it(+-1/4", behind the handle) to help with vapor lock. It still bangs, but only usually when going up and down long steep grades such as in the Sierras or when it gets very hot(100+). I believe it has something to do with the way the tank is hard bolted to the frame and floorpan instead of being strapped like a factory tank.

BTW, I keep another gas cap that I use just for the smog test, which of course stays in perfect condition.