Hello I have a 1968 Pontiac firebird with a 454BBC chevy. I have all new stainless steel 10 gallon tank along with new straps and filler neck, 3/8 stainless steel fuel line (one fuel line going to the carb no return line. I have a holley red electric fuel pump and a holly 750 carb with a pressure regulator 4.5 pressure at the carb. My question is I get a alot of pressure coming from the tank when I remove the vented fuel cap. I did purchase this cap from classic industries and it is the cap that is called for a 1968 firebird. but i get alot of pressure when i take the cap off the tank esp if the fuel guage is low (I know that gas expands) but i think the cap is not doing its job at venting the fuel tank properly. the pressure is soo bad that when i try to take the cap off it litteraly shoots about four to five feet away from the tank and I also get a fumes smell in the vehicle esp if i make a right or left turn and when the fuel is low I am guessing due to the high amount of fumes and pressure in the tank. My question is has any one here had this situation before or does anoyone here in this forum have a part number as a replacement for the OER cap that I bought from classic industries. I have tried Napa Pep Boys Auto Zone Advanced Auto but they give me a cap that does not work I hope that someone in here has a part number that might help me in my situation thanks

Here is what you need to fix your problem. A Pontiac 400...

A 10 gallon tank? Is it a stock 18 gal tank or after market 10 gal. Either way, sounds like you need a vented cap.

Now keep in mind my car is 400, and is has a reproduced standard tank, and reproduced filler tube but this is what it looks like. I have a mech fuel pump at the engine going to a Holley 750 also.

Could it be the fuel pump is pressurising the tank somehow? The amount of pressure need to pop a gas cap that far seems greater than any amount of gas expanding naturally. Or can the smell be coming from the holley? when you smell gas next time pull over and check to see if the top of the carb is wet or the top of the air cleaner lid or in the air cleaner pan itself has wet gas there. It can be the float is set just a bit high and shoots gas up the vent tube....mine has done that.

I know I probably didnt help but just some thoughts.

NPD catalog has a cap like mine, part # C-3028-5, and Classic Ind has a filler tube sleeve that says helps seal off the neck and keep fumes from trunk and interoir

1967-68 Gas Tank to Filler Neck Hose
Seals inlet filler neck to tank stub neck. Eliminate gas
fumes from entering in the interior or trunk area when
properly installed. Produced with correct high quality
neoprene.
K630 1967-68 ..................................... 8.95 ea

Not sure how all this fits your setup since it's not stock

I'm going to wonder out loud about whether you need to go with a fuel return line and a matching 3 line fuel pump.

There has to be a reason that Pontiac went that way with the original V8's.

Have you checked the vented cap? it may be plugged up...? Can you blow thru it?

I have a vented cap too...it works, but sometimes I get a little pressure "poof" when opening it...how bad a pressure is it? too much , if its plugged and you`ll blow the sneding unit gasket if you have a full tank in the summer and gas expands in the heat...at least in Texas heat.

Sounds like you're driving around in a bomb ready to go off. YIKES!!! Seems odd that the less gas in the tank, the more pressure you get unless it's getting really hot.
Better get that fixed fast.

BZs1968firebird my cap looks just like your Mine too is a after market setup. I did check the holley carb that was one of the very first things I did but everything looked great the lines are all dry and the car runs GREAT except for the gas smell. But my vented cap is different than yours but the filler neck is the same setup as mine. I bought my stainless steel gas tank as a kit and maybe the filler neck is for a 400 firebird. I did look at the cap and I am guessing the cap number K630 is possibly a stant gas cap number. I think I saw a cap that looked like yours at pep boys and I will see if its the same number. Thanks for the help

I have seen a few on ebay

Ebay gas cap

Havent seen one that has the black rubber like mine, all have the gold insides. And for some reason they all say "anti-surge" on the bottom, with "vented" on the top. Opposite of mine. Maybe mine is from Australia, isnt everything oppsite down under? ...Joke...mine may not be original, nothing else has been so far...good luck finding something that helps out.

Hello I read your post and clicked on the link on the bottom of the picture could you please tell me what I need that will fix my problem. I am guessing that were going to suggest something in your thread but its not posted. i see that your in flroida I am located in jacksonville florida

i just checked my old vented gas cap and i cant even blow through it then i checked the plunger with a screwdriver pick and pushed the plunger in to check the spring i think its a defective product. i did order a original oe gas cap and i hope this cures the problem

sounds like you found it...