1970 Gas Cap - Vented or Non-Vented

[James Keddie]

The gas cap on my '70 was the vented type and appeared to be original (or at least really old) but when I replaced it with the cap listed in the parts catalog the replacement was non-vented. The gas tank builds up a lot of pressure with the non-vented cap and it has to be removed very slowly, however the car runs fine. Can anyone tell me which is correct? Thank you.

[Terry M.]

Re: 1970 Gas Cap - Vented or Non-Vented

The 1970 gas cap says SEALED in large letters on it. Drawing E 18.2 on page 39 of the third edition of the 1970-72 TIM&JG.

There should be a vent valve on your fuel tank that prevents pressure build-up if it is functioning properly. Perhaps the fuel tank has been replaced, or that valve is malfunctioning. The valve is on the top of the tank, on the right hand side. It will not be visible unless the tank is removed. The valve is a non-replaceable, and as far as I know non-repairable, part of the tank

[James Keddie]

Re: 1970 Gas Cap - Vented or Non-Vented

Terry, Thank you for the response. I will have to take a look at the tank and see if I can tell if it has been replaced with a non-vented one. Since it is building up pressure with the sealed cap, maybe it would be safer to put the vented cap back on?

[Terry M.]

Re: 1970 Gas Cap - Vented or Non-Vented

Depending on how much pressure is building......
Modern vehicles (actually beginning about 1972 or 1973, I think) are designed to build some modest level (a pound or two) of pressure in the fuel tank. In more recent vehicles (after 1995-1996) a test for this pressure is part of the emissions map. Lack of this pressure will set a check engine light or service engine soon light in those vehicles.
I believe I described the operation of the vent on the fuel tank accurately, but before dashing out to make any changes let's give some time for others to chime in here. I know Joe Lucia posted about that vent a while back. I would feel a lot more comfortable having some confirmation of my beliefs.

[Joe L.]

Re: 1970 Gas Cap - Vented or Non-Vented

Terry and James-----

All original late 1969 through 1974 Corvette fuel tanks are equipped with an integral pressure-vacuum valve. This valve replaces the functions of a "vented" fuel cap. The valve can be seen about 6-8" to the left of the fuel filler opening and appears as a quarter-sized disc on the top surface of the tank. Most of the valve is internal to the tank and cannot be seen. The valve is non-serviceable. Generally, if it fails, it will leak and liquid gas will be seen on the top surface of the tank.

This valve, nor vented fuel caps, are designed to relieve pressure or vaccum at ambient pressure conditions. So, tanks will build up some pressure or vacuum before the valve relieves. It is completely normal to experience a "whoosh" when the cap is removed.

If you wish, you can use a vented cap on a 1970-74 model. It will do no harm. However, if the integral pressure vacuum valve is present on the tank (as it should be) and is functioning properly, I would recommend the use of a sealed, 70-74 style fuel cap.

[Terry M.]

Re: 1970 Gas Cap - Vented or Non-Vented

OK, Joe. So it holds some small amount of pressure in the tank.
Hmm. I guess I'll have to see why mine isn't working. I get no pressure build-up.

[Duke W.]

Re: 1970 Gas Cap - Vented or Non-Vented

Joe - did any C2/3 Corvettes use a cap with integral pressure and vacuum relief valves that can be retrofited to earlier models with vented caps?

If so, it might be a better choice than a vented cap as it should help prevent the light end fuel components from evaporating on cars that are in winter storage or don't see much mileage accumulation.

Duke

[Joe L.]

Re: 1970 Gas Cap - Vented or Non-Vented

Duke-----

Yes, the 63-69 "vented" cap is a pressure/vacuum relief cap. It provided pressure and vacuum relief for these cars since the tanks originally installed in these cars had no other provisions for pressure or vacuum relief.

In late 1969 (about August, 1969), the pressure-vacuum valves were added to all Corvette tanks. However, 1969 models continued to use the 63-69 style "vented" cap right through the end of 1969 production.

For 1970, the "sealed" cap was introduced on Corvettes. With this cap, the pressure/vacuum relief function is "transferred" to the integral valve. However, it's my understanding that these caps are not really completely sealed----they will relieve pressure and/or vacuum if the integral valve fails closed and the pressure and/or vacuum gets high enough.

As far as I know, both the 63-69 style "vented" cap and the late69-74 integral pressure/vacuum valves are not designed to relieve pressure and or vacuum to ambient levels. Some build-up of pressure and/or vacuum will occur before the cap or valve relieves down to the specified level.

Also, after about 1970, GM replaced ALL of the 63-69 fuel tanks for SERVICE with tanks which incorporated the integral pressure/vacuum valve. GM instructed that when these tanks were used to SERVICE 63-69 models with "vented" gas caps, the caps should be replaced with the non-vented 70-74 style cap. It is my belief that the incorporation of the pressure/vacuum valve into SERVICE tanks and the instructions to use these tanks with a "sealed" style cap was intended to reduce the intrusion of gasoline fumes vented from the cap into the passenger compartment. Having the pressure/vacuum valve on the top surface of the tank and OUTSIDE of the filler seal ("boot") allows any vented fuel vapors to be vented over and around the tank.

[Dick W.]

Re: 1970 Gas Cap - Vented or Non-Vented

Very good idea Duke. Especially if you are running a racing fuel or blend. The light end aditives are the first to go. I assume that is why racing fuel, unless it is sealed in a drum goes "dead" in a hurry. Especially the newer fuels. The fuels of the 60's and 70's seemed to have a longer "shelf" life.

[Terry M.]

1970 Sealed cap construction

I have restored several 1970 gas caps, and there is no provision for pressure or vacuum relief in the cap. The only components to the cap are the outer shell, a gasket, inner plate, and a spring. The spring is installed between the inner plate and the outer shell (three indentations in the outer ring hold the plate in place). The spring provides the pressure which latches the cap in place when it is installed on the tank, and provides pressure on the gasket to seal the rim of the tank. The inner plate is solid.
While sufficient pressure might build up in the tank to overcome the spring, I wouldn't want to be anyplace around that event. That spring has a lot of pressure, and I suspect the tank would balloon before that spring yielded. Press up on the inner plate of a cap, and you can feel how much pressure it takes.
I have not had the opportunity to examine a disassembled cap from other years. They may be constructed differently.

[Patrick H.]

Re: 1970 Gas Cap - Vented or Non-Vented

Joe,

I'm not sure I agree with the theory of fuel vapor venting location. I can't see much difference (8 inches?) between venting from the new valve or from the tank cap. In fact, the new vent would seem to be in a perfect place to vent right into an open Astro-Vent in the rear storage compartment of a non-AC car if air flow was in the correct direction. Looking at the overflow boots on my cars, they could (sort of) vent vapors down the over flow hose, or the fold-down lid is more than open enough to allow plenty of vapors to escape.

I would suggest that GM just wanted to keep parts numbers to a minimum, and since the new self-venting tank would SERVICE previous years just fine, they used that and then to go along also recommended use of the non-vented cap. I doubt, however, that a vented cap would cause any problems with a vented tank. The integral vent in the new tanks may have been a much better mechanism, however, than venting through the cap.

Patrick

[Daniel Grunwald]

Re: 1970 Sealed cap construction

Thanks for all of the information. My 70 builds enough pressure that I hage to really hold on to the cap when removing it or it will blow off with enough pressure that it could chip the paint on the deck when it pops up from the tank. I never thought much about it but I think I will replace it with a vented cap now.
Daniel

[Jim T.]

Re: 1970 Gas Cap - Vented or Non-Vented

Terry my 70's tank using the original fuel cap has since August 18,1970 always built up pressure. Mayby you don't drive yours enough to build pressure? Then again LT-1's did not have a fuel return line. Perhaps the return line contributes to the pressure build up.

[Terry M.]

Re: 1970 Sealed cap construction

My 1970 has never built up any pressure in the fuel tank that I can recall. Certainly not on the level you are describing. I'll have to think about a way to test that pressure/vacuum valve with a minimum of fuss. One or the other of us has something wrong with our 1970.
I suspect the kind of pressure tester that is used on the cooling system would do the job, the trick is to adapt it to the large fuel tank opening. I think a poor quality gas cap could be adapted, but it will take some ingenuity.

[Terry M.]

Re: 1970 Gas Cap - Vented or Non-Vented

Jim,
That fuel return line sends warmed fuel back to the tank. That warm fuel will raise the pressure in the tank on other than the coldest days.
Yes, I suspect the lack of that fuel return line might lessen the pressure build-up. None-the-less I would expect some pressure in the tank if that pressure/vacuum valve works as Joe describes. And I don't disagree with his description.