Hissing sound from pop open gas cap

[jimbojones 0]

Hello all,
I've had my 69 mach1 for 15 years now, and I just noticed for the first time this weekend a hissing sound coming from the gas cap after parking the car coming back from a normal Sunday drive.  I popped the cap open, seemed to let the air in, popped it closed, hissing sound again.  My immediate thought was that the round 1/8" thick (cork?) gasket that is supposed to seal the cap-to-fuel line had dried off and needed to be replaced.  But after I searched and did not find any such gaskets for sale anywhere (I only found whole gas cap units that include the said gasket, but my unit is otherwise in perfect shape so does not need to be replaced), came these 2 questions : 1- I don't know much about air pressure in fuel line, so is that gasket supposed to be air-tight or instead that hissing sound is normal and I just never noticed it before?  2- if it is indeed supposed to be air-tight and it does need to be replaced, where can I find such a gasket?   Ford?  Autozone?  NPD?  Part number please?  Thank you

[69ShelbyGT350H 61]

More than likely your gas tank is not vented other than the gas cap. As it gets warmer, the gas will expand and push air out, and as it cools, it will contract and suck air in. When you run the engine, it will use fule and again will suck air into the tank to replace the used gas. I know some cars had a vent tube added (factory or dealer) but I do not know the details. More things to research.

[Brian Conway 264]

On my car the filler tube provides the only vent.  Pretty simple way to equalize pressures and pretty much the method used for the 69 cars.  So to answer your question no the tank and cap are not a sealed/air tight unit.  I have not seen the gas cap gaskets sold seperately and the disassemble of the cap seems to be more difficult than it should be.  So that may not be your answer ?  If your tube has the indent, see pictures, hissing would be difficult to achieve and the noise is coming from somewhere else ?   Brian

[2poniesngarage 4]

Lol interesting subject. I have 2 69's one has the flip open cap, like the one pictured above, that has the twist on gas cap inside. Had that car for 15 yrs and have never heard anything from it. My other one just got put together and made the maiden voyage this weekend for a extended trip of say 40+miles got out and heard what u where describing. Now the difference in those 2 are the caps the one with the noise has a twist open cap the large one where the cap itself is one piece. Brand new cap. New gasket. So it would be interesting to hear what yours is. And why there is a difference in those.

[1969_Mach1 333]

My pop open gas cap is NOS Ford, but it's the type Ford issued to correct a safety problem with the first design pop open gas caps.  There was a safety issue with the original type.  They would pop open in an accident and fuel would splash out of the filler.  Ford reissued a second type that on the outside is identical to the original, it pops open, but has a VENTED twist on gas cap on the inside.  The twist on cap comes with a lanyard to prevent loosing it.

Your hissing sound is the vent at the filler is being partially plugged.  Either corrosion or by the gasket. 

[69ShelbyGT350H 61]

5 hours ago, 1969_Mach1 said:

Your hissing sound is the vent at the filler is being partially plugged.  Either corrosion or by the gasket. 

I just searched and found that the only true VENT was added to the 69 Shelby GT500 cars under a recall. I did find that Bob Gaines said the 65/66 caps are vented, the 67/68 are not. No mention on the 69/70 caps. National Parts Depot has several vented caps for the 69/70 Mustangs. NPD's pop open Mach 1 style cap is the type as mentioned that has the twist on cap underneath. My 69 Mach 1 had the original pop open cap, vented.  My 69 Shelby has the twist on cap, vented. Neither have any other hoses or openings for venting so the venting is done via the cap. It seems some 1970 California Mustangs had an emissions tank in the trunk, but no mention on how they were hooked up.

 

Sources : http://www.concoursmustang.com/forum/index.php?topic=11638.msg70294#msg70294

http://www.concoursmustang.com/forum/index.php?topic=16357.msg102403#msg102403

[Mike65 475]

How can you tell if the original pop off cap is vented?.

[69ShelbyGT350H 61]

I'm going to take a guess and say as the Shelby's got a vented cap that replaced the pop open one, and the majority of the repro's are vented, that the original 69/70 caps were vented.

[2poniesngarage 4]

 This is the one that hisses for me. I cant find anywhere in the description of it that says its vented. My other one with the pop open is vented as it is stamped on it like pic above.

www.cjponyparts.com/scott-drake-gas-cap-plain-billet-black-1965-1973/p/GC16/

So anyone know of a way to vent this one? Or is this something to concern one's self with?

[RPM 1,190]

I think the problem is that it's from Scott Drake. Lots 'o crap coming from them lately. 

Several threads floating around where an unvented tank resulted in not enough fuel delivery for the engine to run properly. 

[jimbojones 0]

First of all, thanks for all your answers folks.  I spent all week pondering on your answers, finally set my mind on one answer and solution today, will know at my next ride whether or not my fix corrected the hissing problem.  My theory starts with Brian Conway's answer, which included very detailed pics of what I can tell is a unit exactly like mine, with NO vented twist on cap.  I'm still not sure what material the gasket is made of, but it's some kind of hard cardboard rather than cork but not really relevant at this point, other than the fact that the material is not metal-solid.  The said gasket , after all these years being in place, has been embossed with the form of the fuel tube, including its venting indents, so it has become slowly but more and more air-tight by "shutting" those indents.  My solution is simple, reverse the gasket, other than the "embossing" one one side, it was still in good flat shape on the other side, after I had carefully removed it from the spring plate.  Will come back here to report on the validity of my theory and solution.  If it turns out to be valid, my question of not being able to find and purchase a brand new gasket by itself would still be relevant.  Thanks again.

[LoneWolf2U 136]

On 9/26/2017 at 7:44 PM, jimbojones said:

Hello all,
I've had my 69 mach1 for 15 years now, and I just noticed for the first time this weekend a hissing sound coming from the gas cap after parking the car coming back from a normal Sunday drive.  I popped the cap open, seemed to let the air in, popped it closed, hissing sound again.  My immediate thought was that the round 1/8" thick (cork?) gasket that is supposed to seal the cap-to-fuel line had dried off and needed to be replaced.  But after I searched and did not find any such gaskets for sale anywhere (I only found whole gas cap units that include the said gasket, but my unit is otherwise in perfect shape so does not need to be replaced), came these 2 questions : 1- I don't know much about air pressure in fuel line, so is that gasket supposed to be air-tight or instead that hissing sound is normal and I just never noticed it before?  2- if it is indeed supposed to be air-tight and it does need to be replaced, where can I find such a gasket?   Ford?  Autozone?  NPD?  Part number please?  Thank you

Yes your tank NEEDS to vent, that little ding was put there to allow it and over time the gasket has molded to cover it. Failure to allow venting will cause problems down the road from fuel starving to tank collapse from fuel pump suction.

[jimbojones 0]

Theory verified, not a sound coming out of that gas cap now, I guess no longer gasping for air.  Thanks again for all your advice, but the last question remains : where the f*** do you buy that gasket?  I guess I don't need to know now, the other side should start to behave in 48 some years !