[Hotrodjohn71]

My 1967 Firebird has a non vented tank and the cap on it now is a true vent cap (allows ventilation both ways, in and out) but it is pretty old and worn out. (See photo of old cap)

So I went and got one at Napa which looks similar but apparently has a check valve which will allow vapor to only enter the tank and not exit. It's an anti surge according to Stant.

The issue I have is that pressure is building in the tank when.it sits in my garage which is concerning. It makes a big whoosh sound when I open the cap and I've read of fuel sender gaskets getting blown out

I cannot seem to find another true vent cap which I already have.

Is this tank meant to be under pressure?

[north]

That doesn’t look like a vented cap, it looks like a sealed cap designed for vented tanks.

I don’t know 67-68’s very well but in 69 early cars used non vented caps (vented tank) but later built cars needed vented caps because the factory deleted the tank venting system during the model run. By memory all factory vented caps of the era were anti-surge meaning they would let vapors in and out but not liquid fuel. In practice they all leak gas if you jacked up the front of a car with a full tank.

You absolutely don’t want vacuum building in the tank when you drive, it can starve the engine for fuel on the highway which can overheat the engine and burn valves. Inversely when parked in the sun a sealed tank and cap can spray your face with gas when you remove the cap.

Under extreme vacuum situation in the tank you can actually collapse it .

[Hotrodjohn71]

Thank you North.
The tank itself on my car is non vented. The old cap in the photo is vented for vapor passage both ways.

I checked it by filling it with fluid and the fluid ran out through a widened gap on the handle side.

If the seal in my old cap was not crispy, I'd just keep using it.

So a couple days ago I got a cap at napa supposedly for my 67 non vented tank. I didn't notice but the new cap has a check valve that only let's vapor come into the tank and not out.

That's great except for when I removed the cap, there was a big whoosh of pressure in the tank.

I've read that that much pressure can and has blown out fuel sender gaskets.

I cannot find a cap with a true vent like mine in the picture. All seem to be the same: Vented Anti Surge, which if you research, have a one way only check valve.

So to my origin question for guys with a non vented tank on a 1967 firebird who are not using a factory cap, do you just live with a sometimes pressurized fuel tank, or is there another cap available?

[DM3]

I had the same problem, I took my smallest drill bit and drilled a very small hole in the cap. Problem solved.

[gokitty]

Have seen a small self tapping sheet metal screw installed on the outside of the gas cap. Apparently, there was enough air space around the threads for air pressure to escape but was enough of a plug that fuel did not splash out . Looked wanky, but it worked.

["QUICK-SILVER"]

Originally Posted by Hotrodjohn71

I've read that that much pressure can and has blown out fuel sender gaskets.

Needle and seat in carburetor will let fuel through long before that sending unit seal blows out.

If engine is off when this happens.... Things can get ugly fast when trying to crank and a cylinder is full of gas.

Clay