I hope someone out there can shed a little light on this problem. On the way home to Saint Louis from a show in Eureka Springs last weekend the fuel tank in my 69 collapsed. I have been driving this Vette on occasional roadtrips for the last 5 years and never had any problems like this. I have read quite a few posts on the Forum on the issue but they all seem to point toward using the wrong non-vented gas cap which is not the case here, at least as far as I can tell. I was able to suck air through the cap to the tank side although it was a pretty small trickle of air.
I am running a pretty stock setup with a stock fuel line, stock fuel filter and return line to the tank. It has a L46 350, 355 rearend, naturally Quadrajet aspirated, performance heads and headers.
I have dismantled the 4 year old Paragon repop gas cap to verify that the check valve was not sticking but the interior of the cap looked like new.
We were running a little harder on the highway than we usually do, averaging 75 to 80 mph for some extended lengths of time, but I just can't believe that I was using more gas than the check valve would allow air to displace. But that's the only thing that I can come up with. I have considered adding some venting here somehow but it doesn't seem right that it would be needed.
If anyone has heard of a similar situation or has another theory I'd love to hear it.

Thanks

 

I would try opening or removing the cap next time it happens to see if that's the problem.

 

If your gas cap was venting, the tank could not have collapsed. I have no explanation for why the cap was not venting....but it wasn't.

 

Did a little more checking on things last night. I found the original gas cap that was on the car when I bought it. It does vent considerable more air than the repop. The best I can guess is that the repop wouldn't vent a large enough amount of air to keep up with the amount of fuel being used or intermittently stuck closed. Either way, I am putting the original cap guts in the repop cap lid going back on the new tank.

Thanks for the help.