Gas Tank Vent Hoses How Specific Do They Have To Be

[Freez74]

Doesn't the gas cap vent?

 

240s do.

 

Isn't the cap designed to vent pressure outward instead of allowing air into the tank as fuel is used?

[Kurbycar32]

yes the cap is vented

 

Ultimately it was always vented through the gas cap.  The vapor tank was sealed into the same system when it was in place, it didn't have a vent either. The whole idea of the vapor tank was to accumulate gas vapor, allow it to condense and drip back into the fuel tank instead of venting glacier killing vapor into the atmosphere.  I have nothing against the system but it was broken and I prefer my interior not to smell like gas.  I'm no engineer but I think running gas fumes through the interior of the car is also a terrible idea and potential safety issue.

[Captain Obvious]

The cap is vented, but it's a check valve that only lets air INTO the tank as fuel is used. It will not let pressure out.

 

PRESSURE that's developed in the tank is supposed to vent to atmosphere through either the flow guide valve or carbon canister (depending on the year). If you've capped all the vent hoses and are counting on the gas cap to vent that pressure, you're at risk of bulging your fuel tank or overpowering the float valves on your carbs and flooding your engine.

 

Let's say you've got a quarter tank of gas and your car has been sitting in your cool garage overnight. Then you pull your car out of the garage on a summer morning and then drive it until hot and then park it in the 100 degree sun for the afternoon... Something's gotta give.

 

You need a pressure release somewhere.

[siteunseen]

What about those extruding "U" shaped bumps? What are they for?

[Captain Obvious]

Not sure who your question was directed to or what bumps you're talking about, but if you're talking to me and you're asking about the bumps on the top side of the gas cap, then those are air holes that lead to the atmosphere side of the check valve built into the cap.

 

When the pressure inside the tank gets low enough, the check valve will open and air will be pulled into the tank through those holes. Air isn't ever supposed to flow out of those holes... Only in.

 

If you look carefully at the pic above with the paper clip... The clip is holding the check valve open and you can see the little rubber seal under the disk that the clip is lifting up. Pull the paper clip out and a spring pulls that disk back against the underside of the cap thus holding the check valve closed.

 

And if you're not talking to me or weren't asking about the bumps on the cap, then please disregard this response completely. 

[siteunseen]

Yes, those are the ones (I'm talking to and about.)

The vent is in that manifold up front on the driver's side? I've always thought the cap vented out the pressure. Motorcycle mindset.

Thanks for the reminder, if you were talking to me about those bumps on the the outer edge of the gas cap. 8^)

[Captain Obvious]

The vent is in that manifold up front on the driver's side? I've always thought the cap vented out the pressure. Motorcycle mindset.

 

Yup. That's the one. They called that the Flow Guide Valve, and it's purpose was to vent the gas tank pressure into the crankcase when the engine wasn't running. These vapors would be "stored" there until the engine was running, and then once the engine was running, those vapors were pulled into the intake manifold through the PCV and burned. I'm skeptical as to how effective an engine crankcase is for "storing vapors", and apparently the emissions czars were as well because starting in 74 they went away from the idea of storing the vapors in the crankcase and went to the activated carbon canisters instead.

 

I've not had anything early enough to have a flow guide design (everything I've had is 74 or newer), but I believe the years with the flow guide were supposed to have a non-vented gas cap. The flow guide is supposed to direct air from the air cleaner housing into the tank if the tank draws a vacuum.

 

In other words, the later years used a check valve in the gas caps, but I think the earlier years did that check valve function as part of the flow guide valve.

 

In any event, there still needs to be a way to vent pressure out of the tank.

 

[siteunseen]

Check out these pictures of an unvented tank, amazing.  1st post http://www.zcar.com/forum/10-70-83-tech-discussion-forum/295560-perils-unvented-gas-tank.html 

[Freez74]

That link just goes to the home page for me.

[SteveJ]

That link just goes to the home page for me.

There were a couple of junk characters after the URL. This should work: http://www.zcar.com/forum/10-70-83-tech-discussion-forum/295560-perils-unvented-gas-tank.html

(At least it worked in my preview.)

[Freez74]

It works, reading now.

[Captain Obvious]

That link shows clearly the perils of letting the tank draw a vacuum. It doesn't have any pics of a tank that has bulged outward from pressure, but some of those may be out there as well.

 

In normal operation the tank can draw either a vacuum or develop pressure and you need to have provisions somewhere in the system to account for both of those possibilities.

 

That drawing on the previous page of this thread looks to recommend completely eliminating all the venting in the system. Am I reading that right? Are people really completely eliminating the vents on their systems?