Vented Gas Cap Concern.

[Timothy B.]

Re: Vented Gas Cap Concern.

Jim,

Instead of restricting the exhaust crossover under the manifold try wiring the heat riser valve open, this will accomplish the same thing with much less work and is reversable.

The exhaust will take the path of least resistance and go out the pipe.

[Joe C.]

Re: Vented Gas Cap Concern.

Hi Joe:

I once disassembled a 1967 vented cap, and the construction did not correspond to what you describe. All the parts were metal, and they implemented a simple 2-way valve that provides some degree of resistance to air passing in either direction.

I can imagine that a corroded valve assembly could either leak at low pressure or refuse to vent at high pressure, although I have not actually seen these types of failures.

In terms of leaking at low pressure, I doubt that enough vapor would escape through the small outlet orifice to cause any danger. In my experience with my 67, the main culprit for gas fumes in the garage is the vented fuel bowls on the carb.

Hi Joe,

Here are some photos of the "guts" from my original 1965 cap. It is possible that the design was changed at some point. The first photo shows the inside of the inner chamber from the cap. The flip side of the outer annulus shown receives the paper gasket. The second photo shows a closeup of the valve housing. The third shows the inner parts, clearly showing the perforated disc and the neoprene sealing disc. Last photo shows the newly plated finished product.

Inspection reveals a design which allows a restricted amount of vapor to vent, thereby keeping the tank at atmospheric during normal driving and weather conditions. Any sudden and catastrophic rise in pressure causes the spring to collapse, thereby allowing the rubber disc to seal against the metal disc.

I found corrosion, dirt, and spider's silk in this valve when I opened it up.

[Joe C.]

Re: Vented Gas Cap Concern.

A couple more shots showing the tank side:

[Joe R.]

Re: Vented Gas Cap Concern.

Hi Joe:

I'm away from home right now so I can't check the cap I took apart, but I'm 99% sure that the valve assembly contained only metal parts. I was interested in how the valve worked and how the cap could be disassembled for plating. I have all the parts in a zip-lock bag in my shop.

In thinking about the cap I took apart, I'm not sure it was my original 1967 cap, although it may have been. I can probably figure that out too when I get home.

When I bought my car it had what appeared to be its original cap, but I aquired a couple more at Carlisle later. I'm not sure which one I took apart.

What I recall is that the one I took apart was a rusty example of the correct style for 1967, so I was not too worried about preserving it. I believe GM used the same vented style cap for several years of production, so the internal design of the valve assembly may have changed during that period.

The one you show in your photos is certainly the correct style (vented, small spot welds, smooth rivet, and round SMC logo), but I think the valve assembly differs from what I found. I'll check when I get home.

[Jim T.]

Re: Vented Gas Cap Concern.

Jim,

Instead of restricting the exhaust crossover under the manifold try wiring the heat riser valve open, this will accomplish the same thing with much less work and is reversable.

The exhaust will take the path of least resistance and go out the pipe.

Tim I have wired open the heat riser valve on my 68 and 70 for over thirty years. Mufflers last longer. Good tip, thanks.

[Joe R.]

Re: Vented Gas Cap Concern.

Hi Joe:

I'm back home now and I pulled out the disassembled cap that I had in my shop. Attached is a photo of the parts in the valve assembly.

This particular cap is the one that was on my 67 when I bought the car, and I believe it to be the original cap. It had so much corrosion damage on the underside that I took the lower half out and "sacrificed" it to learn how it was designed.

Originally I thought that all the parts were metal, but there is a rubber/neoprene disc riveted to the back side of the part marked by the arrow. The back of the small metal cap rests against the rubber disc to seal for pressure applied from inside the tank.

From your photos it looks like your cap uses essentially the same construction, although the rubber disc in your cap is not riveted to the metal cap.

In any event, I think the operation of my cap uses the exact same concept as a typical radiator cap:

1) The small spring, small cap, and rubber disc implement a one-way valve that allows atmospheric air to enter the tank after enough vacuum develops in the tank to overcome the pressure of the small spring.

2) The large spring, the large underside piece of the cap assembly, and the large seal around the perimeter of the cap underside implement a second one-way valve that allows the tank to vent to the atmosphere if enough pressure builds up inside the tank to overcome the large spring.


I do not see any aspect of the construction that would serve to shut off the flow once either valve opens. The flow of atmospheric air into the tank is limited by the small orifice of the center hole where the small valve is riveted to the bottom piece of the cap. The available flow from inside the tank to the atmosphere is quite large once the force of the large spring is overcome, since that flow is around the entire perimeter of the large cap seal.

[Joe C.]

Re: Vented Gas Cap Concern.

[ATTACH]19263[/ATTACH]Hi Joe:

I'm back home now and I pulled out the disassembled cap that I had in my shop. Attached is a photo of the parts in the valve assembly.

This particular cap is the one that was on my 67 when I bought the car, and I believe it to be the original cap. It had so much corrosion damage on the underside that I took the lower half out and "sacrificed" it to learn how it was designed.

Originally I thought that all the parts were metal, but there is a rubber/neoprene disc riveted to the back side of the part marked by the arrow. The back of the small metal cap rests against the rubber disc to seal for pressure applied from inside the tank.

From your photos it looks like your cap uses essentially the same construction, although the rubber disc in your cap is not riveted to the metal cap.

In any event, I think the operation of my cap uses the exact same concept as a typical radiator cap:

1) The small spring, small cap, and rubber disc implement a one-way valve that allows atmospheric air to enter the tank after enough vacuum develops in the tank to overcome the pressure of the small spring.

2) The large spring, the large underside piece of the cap assembly, and the large seal around the perimeter of the cap underside implement a second one-way valve that allows the tank to vent to the atmosphere if enough pressure builds up inside the tank to overcome the large spring.

I do not see any aspect of the construction that would serve to shut off the flow once either valve opens. The flow of atmospheric air into the tank is limited by the small orifice of the center hole where the small valve is riveted to the bottom piece of the cap. The available flow from inside the tank to the atmosphere is quite large once the force of the large spring is overcome, since that flow is around the entire perimeter of the large cap seal.

Joe:

In my cap vent, the rubber seal is not riveted to the metal "piston" which is marked in your photo. If it was, then it would significantly change the operation of the vent system. It is possible that a design change was implemented sometime between May 1965, and the time your '67 was built. In mine, you can see that the vent housing (extreme left in your photo) is riveted to the conical inner half of the gas cap. If your photo was of mine, then your arrow would be pointing directly to the neoprene seal, and then, to the right of that (between the light spring and the seal), would be another metal disc which locates the neoprene seal between the spring and the smaller metal disc (second from left in your photo).

The mechanism which "checks" the vent against sudden high pressure rise in the tank, is high outbound flow, which will cause the small disc with the 2 semicircular holes cut into its sides, (pictured immediately to the left of the arrow in your photo) to move and seal against the neoprene rubber disc. The small disc resides with its vertical edges fitting within the annulus which is formed in the roof of the vent housing, which is in turn, riveted to the inner cone of the gas cap. The small metal disc should be free to move, and any debris which impedes this movement will disturb the design function of the cap. On a refurbished and properly assembled cap, the small disc with the semicircular holes on its sides, should be heard rattling around inside the vent housing if the cap is shaken.

Negative pressure within the tank is free to equalize at all times, via the 2 semicircular holes cut in the sides of the small cup shaped metal disc. In other words, air is free to enter the tank as fuel is depleted.

Small-to-moderate amounts of air are free to escape the tank, via those same two semicircular holes cut into the sides of the small cup shaped metal disc which resides within the annulus in the roof of the vent housing which is riveted to the inner cone of the cap.

If a large and sudden pressure rise is experienced within the tank, those two small holes cut into the sides of the metal cup shaped disc will choke the flow. This causes a pressure rise within the small cup shaped metal disc, which then moves toward the neoprene disc, and seals against the small hole which is in the center of it. This then seals the tank against internal pressure, and transfers "control" to the heavy, outer spring, which then works the same way as a radiator cap in relieving the tank of pressures approaching 10-15 psig.

It can be seen how important it is that the valve assembly be clean and corrosion-free. It is also important to see that the valve will only work properly if the small cup shaped disc is free to move, AND that it is oriented as installed on the tank. If the cap is turned upside down, it will not work properly.

Joe

[Joe R.]

Re: Vented Gas Cap Concern.

Hi Joe:

Thanks for your detailed explanation. I pretty much agree with your analysis. What was interesting to me is that the small valve is actually open to limited flow in either direction, which makes it different from a radiator cap. I tried the "rattle test" that you describe, and indeed the small disc is free to move up and down through a limited range without overcoming the spring.

Regarding the rubber seal, attached is a photo of the one from my presumed-1967 cap. The rubber disc is riveted to a metal cap that rests against the small spring. There is a decent size hole in the rivet center, although pressure from inside the tank could lift the free-floating metal cap and seal this hole, transferring the load to the small spring.

I guess what this implements is an open, bidirectional vent for small air flows, but a mechanism to close that vent if a sudden rush of air or fuel tries to exit the tank. If the vent closes and pressure continues to rise, there is a 2-stage release of first the small spring and then the large one.

It's an interesting mechanical contraption that apparently tries to control the flow for a variety of situations. Maybe these included fuel sloshing against the cap and/or collision damage.

[Joe C.]

Re: Vented Gas Cap Concern.

Hi Joe:

Thanks for your detailed explanation. I pretty much agree with your analysis. What was interesting to me is that the small valve is actually open to limited flow in either direction, which makes it different from a radiator cap. I tried the "rattle test" that you describe, and indeed the small disc is free to move up and down through a limited range without overcoming the spring.

Regarding the rubber seal, attached is a photo of the one from my presumed-1967 cap. The rubber disc is riveted to a metal cap that rests against the small spring. There is a decent size hole in the rivet center, although pressure from inside the tank could lift the free-floating metal cap and seal this hole, transferring the load to the small spring.

I guess what this implements is an open, bidirectional vent for small air flows, but a mechanism to close that vent if a sudden rush of air or fuel tries to exit the tank. If the vent closes and pressure continues to rise, there is a 2-stage release of first the small spring and then the large one.

It's an interesting mechanical contraption that apparently tries to control the flow for a variety of situations. Maybe these included fuel sloshing against the cap and/or collision damage.

That's right, Joe.....an "elegantly simple" device.

Your riveted seal is different from mine but appears to have the same function. The rivet looks to be recessed, and the sealing surface where the disc contacts the neoprene is just outside the rivet. "Bypass" air flows through the rivet hole when the valve is not "checked".

Kudos to you for having the analytical skill to translate my detailed and convoluted explanation and applying it to the two gas caps.

Joe

[Michael F.]

Re: Vented Gas Cap Concern.

in a previous garage I had a small exhaust fan low down near garage door, worked for me