1970 Gas Cap - Vented or Non-Vented

[Joe L.]

Re: 1970 Gas Cap - Vented or Non-Vented

Patrick-----

There may have been some element of parts consolidation that played into this. However, if that were the only rationale, then why specify that for 63-67 models the tank should be used with the "sealed" cap? The tank with the integral pressure vacuum valve can be used with "vented" cap (late 1969's, including mine, came from the factory that way), so why specify that the cap be changed when the updated tank is used in SERVICE as a replacement?

Also, the fuel tank filler "boot" is described by GM as a SEAL. It is designed to "seal" against the bottom surface of the deck lid and around the filler neck. Fuel vapors which vent from the cap then have to escape from either the drain or the unsealed filler door opening. I'm sure that most vent through the filler door opening. For C3s, especially C3 convertibles, the air flow created by the "kammback" design causes these venting vapors to be carried into the passenger compartment. I believe that's one of the reasons why GM added the integral pressure/vacuum valve in late 1969 (the other reason was the upcoming EEC controls first used for 1970).

[Duke W.]

Re: 1970 Gas Cap - Vented or Non-Vented

Joe - just ran a pressure and vacuum test on what I know is my original '63 fuel cap and it holds neither pressure nor vacuum, so I believe it is just a "vented" cap with no valve of any kind. Since you probably have both the vented and non-vented type in inventory, maybe you can check them.

If the later cap does actually have pressure and vacuum relief valves, it might be worth retrofiting to early models, even if they don't have the valve assembly on the tank, but it would be nice to know what the pressure and vacuum relief values are.

I replaced my original tank circ 1977 and I cannot find any evidence of a valve between the fill neck and either of the two retaining straps. It's a little tough to see the very rear part of the tank, but since my "boot" isn't installed at the current time, I could get a pretty good view of most of the top of the tank inboard of the retaining straps with a strong flashlight.

It's interesting that with new cars having such low tailpipe HC emissions, evaporative emissions even from modern cars with evap control systems are estimated to be nearly half the total HC emissions from cars.. Even though our vintage cars have significant tailpipe HC, since many are driven very little, evap emissions may exceed those from the tailpipe, and the first to go are the light end components, which generally have the highest octane rating, so eventhough "old gas" may not go bad from gum and varnish formation, it can loose octane if stored in a non-sealed container, which is essentially what a fuel system with a vented cap is.

Duke

[Patrick H.]

Re: 1970 Sealed cap construction

"One or the other of us has something wrong with our 1970."

Yours doesn't get driven?

Patrick

[Joe L.]

Re: 1970 Gas Cap - Vented or Non-Vented

Duke-----

The early 1963 cap, GM #3820865, is different than the later 63-through 67 cap, GM #3843576. The 3820865 has only a small hole for ventillation. The 3843576 has a pressure/vacuum valve.

The 1968 cap, GM #3921689, is also a cap with a pressure/vacuum valve which became the SERVICE cap for 63-67 Corvettes after July, 1968 when the former cap was discontinued from SERVICE. I don't know how the later cap differs from the former. Visually, they are identical. I expect that some change was made to the pressure/vacuum valve specs.

The 1969 cap, GM #3952708, also has a pressure/vacuum valve. It became the SERVICE cap for 1963-67 Corvettes and 1968 Corvettes when the GM #3921689 was discontinued in May, 1969. The 3952708 has a rubber gasket whereas the earlier caps use a cellulose-fiber gasket. I don't know if there are any other differences; none are apparent, anyway.

I will check the pressure/vacuum spec of the 1969 cap when I can dig one out. It would be very interesting if someone could do the same for known 64-67 caps and the 1968-only cap.