How Does a Vented Gas Cap Work?

[kbuhagiar]

Hello, folks,

Should the vented gas cap on our Corvettes relief pressure in both directions? Or is it a one-way relief valve?

I opened the gas cap three times yesterday afternoon, and three times I experienced a substantial relief of air pressure from the tank (like opening a can of soda, only smellier). Car was parked in the garage, and had only 2 or 3 gallons of gas in the tank. Gas cap is brand new and vented.

I just got the car running again after 24 years, so I am not sure if this is normal or not. Furthermore, I am experiencing a hard-start problem when the engine is warm, so I am wondering if the two symptoms may be related (excessive pressure in the fuel system pushing fuel past the needle & seat in the carburetor).

Thanks in advance for your assistance.

 

[WayneC]

Hello, folks,

Should the vented gas cap on our Corvettes relief pressure in both directions? Or is it a one-way relief valve?

Does the word "vented" appear on the top of the cap? Is it an original cap or a repro?

I can't answer definitively, but I would expect it to vent in both directions.
When the fuel in the tank heats up or evaporates, pressure will build in the tank if the tank doesn't vent out (especially if the fuel level is low). When the engine draws fuel from the tank, a vacuum will form in the tank if it doesn't vent in. I would think our midyear Vette caps should vent in both directions. If your cap is a repro vented cap, I'd suspect the vendor of the cap got the design wrong and it only vents in, because emissions laws that came in during the 70's probably required that the cap not vent out... only in. In place of a 2-way vented cap, many US cars of the 70's ran a line from the tank to a charcoal filter to vent vapors.

'68-up Vettes have vented fuel tanks that are virtually identical to the early tank except they have one or two built-in vents depending on year (and I suspect they are "in-only" vents); unlike 63-67, they use a cap marked "sealed"... I think either cap fits both but unless a vented cap has a 2-way vent, it may not do the job correctly for midyears.

One possibility is that the vent on your cap is blocked or it has a two-way valve that's frozen in one position; if not, a quick workaround may be to keep the tank topped off (or drill a tiny hole through the cap and don't ever park on a steep hill with a full tank ).

 

[twiget]

I'm pretty sure a vented gas cap is basically a check valve. It allows atmosphere into the tank as the level decreases, so the tank does not draw a vacuum, and keeps the gas vapors in the tank where they belong.

The vapor pressure of gasoline being what it is, if a vented gas cap allowed 2-way flow, most of the gas in the tank would evaporate.

Jason

 

[kbuhagiar]

Does the word "vented" appear on the top of the cap? Is it an original cap or a repro?

The word "vented" is embossed in big letters on the cap. It is a brand new repro cap, purchased two months ago from a local Corvette vendor.

My follow up question: Can enough pressure build up in the system to push fuel past the needle & seat valve in the carburetor (brand new Edelbrock 1406)?

 

[sub006]

One possibility is that the vent on your cap is blocked or it has a two-way valve that's frozen in one position; if not, a quick workaround may be to keep the tank topped off (or drill a tiny hole through the cap and don't ever park on a steep hill with a full tank ).

My dealer-supplied chrome locking C2 caps all had that tiny hole leading to a 1/2" pipe on the inside, facing down. EPA wouldn't approve, but that's what the original unvented tank wants!

 

[kbuhagiar]

My dealer-supplied chrome locking C2 caps all had that tiny hole leading to a 1/2" pipe on the inside, facing down. EPA wouldn't approve, but that's what the original unvented tank wants!

Funny that you mention the locking gas cap - that's exactly what I had before, and I do not remember ever experiencing the pressure problem with it.

The only reason I replaced it is because it just plain wore out and was giving me problems, so I bought a new (repo) standard cap.

I am seriously considering going back to a locking cap, because, based on my informal research, everyone who has a locking cap doesn't seem to have a problem with pressurization of the tank...;squint:

 

[kbuhagiar]

I'm pretty sure a vented gas cap is basically a check valve. It allows atmosphere into the tank as the level decreases, so the tank does not draw a vacuum, and keeps the gas vapors in the tank where they belong.

The vapor pressure of gasoline being what it is, if a vented gas cap allowed 2-way flow, most of the gas in the tank would evaporate.

Jason

Thanks for the reply.

After further research, I found a reference in Noland Adams' restoration guide (page 94, I believe) describing the 63-67 vented gas cap as "able to relieve both pressure and vacuum".

Hmmm...my brand new cap releases vacuum, but won't relieve pressure!...Am I mis-interpreting Noland's description?

And I can live with the pressure (I guess), so long as I know that it won't push fuel past the inlet valve in the carb....

 

[sub006]

When I "Grand Sported" my car in the early '80s, I went back to my 20-year-old original cap, since I could now lock the trunk lid over the tank! It worked fine for 10 years or so. Car's been in storage 15 years now, so don't know if the cap's still good. Will probably convert to a stainless fuel cell tank anyway.

 

[JohnZ]

After further research, I found a reference in Noland Adams' restoration guide (page 94, I believe) describing the 63-67 vented gas cap as "able to relieve both pressure and vacuum".

That's correct. It only vents internal tank pressure above a certain pressure, so it's not open to the atmosphere; it also opens at a certain vacuum level (below atmospheric pressure) to allow air to replace the volume of fuel used.

The dealer accessory locking cap didn't have that valve - it was just open to atmosphere through a small hole visible in the top of the cap.

 

[kbuhagiar]

That's correct. It only vents internal tank pressure above a certain pressure, so it's not open to the atmosphere; it also opens at a certain vacuum level (below atmospheric pressure) to allow air to replace the volume of fuel used.

The dealer accessory locking cap didn't have that valve - it was just open to atmosphere through a small hole visible in the top of the cap.

John - thanks for the reply.

Could you venture on opinion as to whether the pressure that builds up using the standard cap would be enough to push fuel past the inlet valve in a carburetor (leading to a hard start condition when hot)?

I read elsewhere of that being a problem with the Holley 3x2 setup on the big blocks...supposedly the fix was to drill a tiny hole in the gas cap...:ugh