Fuel System: Passing Gas

[LeoH]

I love coming up with new thread titles, can't you tell?
I notice after driving home in the afternoon heat, 80s, about 10 miles of city driving, that I get a hissing from the gas tank out the cap in my 60 Lark VI wagon after I stop the car in the garage.

Is that normal? Even if it is, can I get a new rubber flap for my gas cap and is it as easy to replace as it appears...just unscrew the two screws and replace?

[JoeHall]

I see you are in Nevada, but must be somewhere cool since the temps are only 80s. The hiss is pressure build up,by heated, expanding gas. THE PROBLEM: the gas cap is not vented to "blow" outward when gas expands. Eventually the pressure will blow through somewhere: at the gas gauge sending unit, the rubber hose coming off the metal "nipple" at the bottom of the gas tank, or swell the tank, causing leaks at the seams. I once had about another 1/2" left before the above rubber hose woulda blown off. I noticed it while stopped at a gas station somewhere in Arizona, years ago.

THE FIX is simple: vent the cap, or replace it. Replacement caps are available from our vendors, that blow and suck, at very low pressure/vacuum, the way S-P intended. Most modern caps only suck, but do not blow. Another fix is to drill a hole in center of the cap, about .050" diameter. Be sure to drill all the way through the vent valve. Also, you can take a razor and cut a 'V' slit in the cap seal, about 1/16" wide, so the pressure escapes through that slit. Any of these remedies will cause a mild gas smell when making hard right turns with the tank 3/4 full or more. Not a big deal though.

One more remedy, is to leave the modern cap in place and run a vent line from the filler neck to a mid-late 1970s vintage charcoal cannister. That is what I did to the two Hawks with HFI; no pressure and no smell.

Its a simple problem with several simple fixes, but it could get real ugly if not addressed, especially in hot weather.

[LeoH]

Thanks Joe. For the time being, just cracking the cap after I park, will that do it? As for drilling the hole, I see there's a hole in the center of the cap, and then another hole to the shoulder, is that supposed to be doing the venting, in theory?

Would installing a new piece of rubber, instead of the black, hard former rubber gasket do anything? I'll give the razor on the former rubber piece step for a first go around.

[JoeHall]

Thanks Joe. For the time being, just cracking the cap after I park, will that do it? As for drilling the hole, I see there's a hole in the center of the cap, and then another hole to the shoulder, is that supposed to be doing the venting, in theory?

Would installing a new piece of rubber, instead of the black, hard former rubber gasket do anything? I'll give the razor on the former rubber piece step for a first go around.

The holes already in your cap may be for venting, but if its building enough pressure to hiss, they are not venting as they need to. Problem with loosening the cap when stopped is that it builds the same amount of pressure when moving. The razor is simplest, but very effective. Be sure to locate the slit on the high side of the seal, in relation to the filler neck, if you know what I mean. Some seals will rotate around and some will not.

[LeoH]

I did do the slit trick and I did get turned around with the up side when I took the collar out so it is slit on the upper part instead of below the collar as I planned. Best laid plans 'n all. I will get a newer vented cap and use this for a backup after the other arrives. It did make a difference today after I parked in the garage, thanks.