66 Sealed Gas Cap

[Mark P.]

I just got a new gas cap for my 66 and it is a vented gas cap (as it should be per the JG). When I pulled the old cap off, it says sealed on it. I didn't know a sealed cap would work on a car that was supposed to have a vented cap. I haven't experienced any isues over the past year and I've driven it quite a bit.....what are the potential problems from using a sealed cap on a car that should have a vented cap? Perhaps the sealed cap wasn't sealing very well, so that's why it worked? Any input is appreciated.

Thanks,
Mark

[Joe L.]

Re: 66 Sealed Gas Cap

Mark------

Since November, 1970, all GM SERVICE fuel tanks for 1963-1969 Corvettes have incorporated an integral pressure-vacuum valve. This integral valve makes a vented fuel cap unnecessary and GM instructs that when these tanks are used in any 63-69 model Corvette, a 1970-74-style "sealed" fuel cap should be used. It's not absolutely critical, but it's actually an advantage to use the sealed cap on the road since it minimizes the intrusion of fuel odors into the passenger compartment. One can always install a correct fuel cap for judging; it's a pretty easy swap-out.

So, how can you tell if your car has the tank with the integral pressure-vacuum valve? There are 2 ways; one positive and the other nearly positive.

The positive way is to look to the left of the fuel filler on the top of the fuel tank. You need to pull the filler boot aside to do this. If, about 6" to the left of the fuel filler you see a round, more-or-less flat device flush with the tank, then you have the integral pressure-vacuum valve.

The other way is to look at the tank from under the car. You should be able to see the very ends of the tank above the spare tire carrier. Look at the major depressions on either end of the tank. If they are "round", then the tank does not have an integral pressure vacuum valve. If they are more-or-less "rectangular", then there's about a 99% chance that the tank DOES HAVE the integral pressure-vacuum valve. There were some tanks manufactured with the "rectangular" depressions and without the valve, but it's VERY unlikely you have one.

[Mark P.]

Re: 66 Sealed Gas Cap

Thanks, Joe. I have an original tank for my '66, and so I figure it should have the pressure vacuum valve in the top of the tank like I have seen in others I have replaced. I guess I should have answered my own question. It didn't make sense that it would have the tank without the valve and a sealed cap. That wouldn't work. I'll double check for the valve tonight. Thanks for your patience and advice.

Mark

[Bill S.]

Re: 66 Sealed Gas Cap say what?

i was checking my 66 tank after seeing this post. the way i read it is that it is original and non vented if it has the round depressions. the next post says mine is an original tank so it must be vented. can some one clarify this for me, thanks,bill

[Joe L.]

Re: 66 Sealed Gas Cap say what?

bill-----

Original 1966 Corvette fuel tanks did not have an integral pressure vacuum valve. They have to be used with a cap that says "VENTED" on it. This cap happens to have a "scalloped" perimeter (but, 1970-71 "sealed" caps do, too, and they'll fit perfectly on a 1966; so the important thing to look for is the embossed "VENTED").

GM (or, aftermarket) REPLACEMENT fuel tanks for 1963-69 Corvettes purchased after 1970 have an integral pressure-vacuum valve. GM isntructs that they be used with a "SEALED" type fuel cap. However, they can be used with a "VENTED" cap if one so desires. I recommend the "SEALED" cap for these tanks except for "show" purposes.

I agree that the post you refer to was confusing.

[John H.]

Re: 66 Sealed Gas Cap say what?

Bill -

Midyear fuel tanks were NOT vented, so the cap WAS vented, and "VENTED" is stamped on the cap.

[Bill S.]

Re: 66 Sealed Gas cap understood Joe's

Joe i understood your post. it was the post Mark left. i got the impression that because he felt his tank is original it must be vented unless he meant the cap. i wanted to revisit in the event he missread it. bill

[Joe L.]

Re: 66 Sealed Gas cap understood Joe's

bill----

Yes, it was Mark's post I was referring to. It was somewhat confusing. The ORIGINAL 1966 tank was not equipped with an integral pressure-vacuum valve. All pressure-vacuum relief was provided by the cap. I'm 100% certain of that.