I have a 69 Camaro (Big Block 396), and had issues with the sending unit. The gas tank was all dented from the bottom and was cratered at the top which caused the sending unit to get out of wack with incorrect fuel reading. I thought the tank was just damaged so I hammered out the dents and put a new sending unit in. All was fine driving for a day or two, and then the fuel gauge was reading incorrectly again. When I looked at the gas tank it was all dented in again. That's when my mechanic and I noticed there was no air vent on the gas tank and that's what's causing the compression. The cap is the "Vented Anti Surge" type (see below), but it seems air tight to me, and this wouldn't happen if it were (would it?). He cut a slice out of the washer just to air could get in, but that's just a temporary fix. He recommended an option of drilling a hole in the neck and attaching a tube so it could breathe, but I don't like that idea. I also read that the cap should be vented on this type of fuel system, but have also seen some say the venting takes place within the line from the gas to the engine somewhere? I see only one line from the tank. Could it be that it's blocked in the line somewhere which is causing the issue, or does this situation revolve around the cap only? Why would a "vented" cap not be vented? If you need actual photos let me know.

Thanks for your help.

 

A gas cap for a 69 Camaro has a "one-way" vent: It will vent out but will not allow air in.

I believe that big block cars had a 2 line fuel sender; one tube was used for a vent.
So it sounds like you may have the wrong fuel sender.

 

Nope, the gas cap vents IN, not out....that is, one that is working as it should. If not, you would get vapor lock and the fuel pump would stop being able to suck fuel, and/or the gas tank will try to turn itself inside out, that sounds familiar. I would buy a different brand cap, yours is defective. And not a generic one from Autozone etc, but from one of the Camaro/GM resto supply companies.
With a 2 line sending unit, the second one was a return line from the front mounted fuel filter, to keep fuel moving (cooler), and to prevent heat soak pressure to build up against the needle and seat in the carb after the engine was shut down. It wasn't a "vent".

 

So either of these two would be the right one to get? Both say vented, one says Canada Model, other says Except Canada Model:

CST/Fuel Cap 6046- ReadReviews onCST #6046



CST/Fuel Cap 6806- Read1Reviews onCST #6806