C2 vented gas cap

[Ed S.]

Need advice / opinion regarding the gas cap on my restored 64 convertible. I removed the gas cap today to add some gas - the car was up to normal operating temp and air temp was around 70 F. When I removed the cap there was a release of pressure - not a great deal but enough to be noticed. Like opening a twist bottle of Pepsi when it is warm. It did not spray gas liquid out and the tank was a little less than 1/4 full. I only added 3 gallons of gas then drove home (about 6 miles). Removed the gas cap again to see what might happen - same thing - there was some pressure built up in the tank and it was released when I removed the cap.

I'm confused. I thought this car had a vented gas system. My gas cap (appears to be original) is stamped "Vented" - see the attached photos (both sides).

A few other details.
- I acquired the car in late August of this year.
- I have always noticed the odor of gas fumes in the garage after the car has been driven - after a while the odor of gas goes away- I understand this is normal with 10% ethanol gas (93 Octane) which I use.
- There are no gas leaks anywhere -all fittings and connections are dry and clean.
- The car was restored in 2004 by a PO. The gas tank looks like it might have been replaced in 2004 - the inside is it is very - very clean. (If the tank was replaced I'm wondering why the PO would use the old cap - aren't new tanks supplied with new caps?)
- The cap looks to be original, the metal (aluminum) is oxidized and the back side is pitted. I pressed my lips against the back side over the vent hole and applied pressure - some air did move through the hole so the cap does not appear to be clogged.

If this is a vented system why am I getting pressure build up in the tank? This concerns me as pressure in the tank will find a weak point and could result in a major leak - or worse.

[Edward J.]

Re: C2 vented gas cap

Ed, I would say some pressure is Ok as the car is driven and gas movement in tank and temp. combined will cause some pressure, I know the caps do have a pressure release, not really sure what there set at on our older corvettes, but A new cap maybe in store for your car. and save the old one and use for judging reasons, That's what most do with my corvettes.

[Ed S.]

Re: C2 vented gas cap

Ed,
Thanks for the quick reply - your explanation sounds logical. A follow-up question for you (or anyone that cares to reply). Regarding the "venting function" on the cap or for the tank, is the vent to allow air in so when the fuel is used and the level gets low it does not create a vacuum that stops the flow of fuel or is the vent intended to release (high) pressure? Essentially, I am wondering if there is a one way or two way relief valve in the cap? Just wondering here - I always assumed that the vent function was to let air in but not let fumes out.

[Joe L.]

Re: C2 vented gas cap

Need advice / opinion regarding the gas cap on my restored 64 convertible. I removed the gas cap today to add some gas - the car was up to normal operating temp and air temp was around 70 F. When I removed the cap there was a release of pressure - not a great deal but enough to be noticed. Like opening a twist bottle of Pepsi when it is warm. It did not spray gas liquid out and the tank was a little less than 1/4 full. I only added 3 gallons of gas then drove home (about 6 miles). Removed the gas cap again to see what might happen - same thing - there was some pressure built up in the tank and it was released when I removed the cap.

I'm confused. I thought this car had a vented gas system. My gas cap (appears to be original) is stamped "Vented" - see the attached photos (both sides).

A few other details.
- I acquired the car in late August of this year.
- I have always noticed the odor of gas fumes in the garage after the car has been driven - after a while the odor of gas goes away- I understand this is normal with 10% ethanol gas (93 Octane) which I use.
- There are no gas leaks anywhere -all fittings and connections are dry and clean.
- The car was restored in 2004 by a PO. The gas tank looks like it might have been replaced in 2004 - the inside is it is very - very clean. (If the tank was replaced I'm wondering why the PO would use the old cap - aren't new tanks supplied with new caps?)
- The cap looks to be original, the metal (aluminum) is oxidized and the back side is pitted. I pressed my lips against the back side over the vent hole and applied pressure - some air did move through the hole so the cap does not appear to be clogged.

If this is a vented system why am I getting pressure build up in the tank? This concerns me as pressure in the tank will find a weak point and could result in a major leak - or worse.

Ed-------


Very normal. The 64-69 caps do contain a pressure/vacuum valve. However, the valve does not relieve to ambient pressure. So, depending on conditions, there is usually an "outrush" of air or an "inrush" of air when the cap is removed.

[Donald H.]

Re: C2 vented gas cap

I can't explain what is happening. But I would suggest you purchase a new reproduction cap. Paragon has them for $30. I bought one to use on my 66 and sent the original off for restoration. I am saving the original for when and if I decide to get the car judged.

Don

[Ed S.]

Re: C2 vented gas cap

Thanks again to all - will definitely get a new cap, and save the original. Most of all, I am relieved to hear from multiple respondents that the slight pressure rush is normal and I do not have a malfunctioning fuel system that is an disaster waiting to happen.

[Stuart F.]

Re: C2 vented gas cap

Ed;

I played this same game with my 63. It originally is furnished with a cap that has a simple single vent hole (63 only). When I tried to replace it (for looks), I found the replacement was the 64-69 pressure/vacuum valve cap like yours, so I ordered one from my local Chevy dealer (yes, they are still available for about $24.00). This I also justified changing in part due to my search for a way to cut down on the gas stink in my attached garage. I too noticed a rush of pressure release when I removed this cap, and Joe explained this for me as well.

On the gas smell; as long as you have a vented system of any sort you will Probably have some level of gas smell, even if you change to a non-ethanol gas as I have done. I have tried and modified many things to improve this situation and determined I have it as good as it will be. I did eliminate my heat soak problem, for the most part, so I am able to start up under most any severe ambient conditions.

Stu Fox

[Ed S.]

Re: C2 vented gas cap

Thanks Stuart
A genuine GM part from a Chevy dealer is more appealing to me than a cap from one of the Vette specialty vendors, even though a lot of their stuff is pretty good.

It's amazing, the things we (vintage Vette owners) put up with just to have a piece of very attractive mechanical history in our garages. I think it's worth the trouble.

Thanks again to all

[Joe L.]

Re: C2 vented gas cap

Thanks Stuart
A genuine GM part from a Chevy dealer is more appealing to me than a cap from one of the Vette specialty vendors, even though a lot of their stuff is pretty good.

It's amazing, the things we (vintage Vette owners) put up with just to have a piece of very attractive mechanical history in our garages. I think it's worth the trouble.

Thanks again to all

Ed------

A new cap from GM is GM #3952708. It currently carries a GM list price of about 77 bucks.

As far as the "whoosh" goes, though, the new cap is going to be exactly the same as the one you have now.

[Leif A.]

Re: C2 vented gas cap

Ed,

Read this response from Joe Lucia in a post of mine from several months ago. His post #3. I only bring this to your attention because you mention that you probably have a replacement gas tank in your car. https://www.forums.ncrs.org/showthre...320-67-Gas-Cap

[Ed S.]

Re: C2 vented gas cap

That's what I love about this forum - and the people that contribute to it. You learn something new everyday. Thanks Joe - did a bit more research and as I understand, among other things the original C2 gas caps has a small "SM" stamped on the back side. My cap, although old and dripping with patina, does not have the SM marking. Thus I assume my cap is not an original - but it does function as one. Plan - acquire an original - I found one site selling them for just a bit less than the $77 Joe quoted.

[Ed S.]

Re: C2 vented gas cap

Leif,
Thanks - I was typing a response to Joe (above) as your response came in. I agree with you - I don't believe my cap is an original as it lacks the SM logo. Thanks for the referral to your earlier thread - wish I had found it sooner.

[Jim S.]

Re: C2 vented gas cap

Ed------

A new cap from GM is GM #3952708. It currently carries a GM list price of about 77 bucks.

As far as the "whoosh" goes, though, the new cap is going to be exactly the same as the one you have now.

Joe,

Is that the same # for a 72 cap?

Jim

[John M.]

Re: C2 vented gas cap

Ed,
I had a vented cap 'fail" so to speak, on my 69. Was driving in the summer and when I slowed down to go into the driveway the car started to bog down and then quit. It wouldn't start and had a strong gas smell. Opened the hood and there was a lot of gas on the intake with more coming out of the carb. What the .......? Finally decided to check gas tank and there was a major woosh, like in big pressure. The leak stopped and after sitting flooded for a couple hours the car started with no gas cap and all was good. Apparently the vent thing froze up closed. Got another cap and sent original to Joe Perri. Scary to think of what could have happened. 40 year old stuff can be dangerous.
John

[Joe L.]

Re: C2 vented gas cap

Joe,

Is that the same # for a 72 cap?

Jim

Jim------


No, the 1972 cap is GM #3994339. It's still available from GM, too, and it even costs less than the earlier cap.

The 3994339 should also be used with 63-69 Corvettes that have been fitted with a GM SERVICE fuel tank with integral pressure/vacuum valve. I highly recommend the use of a fuel tank with integral pressure vacuum valve and 3994339 cap for ALL 1963-69 Corvettes. No one will ever see the integral pressure/vacuum valve and the cap can be replaced with an original cap for judging only. I mean, how difficult is doing that?