72 Brave gas tank vacuum (?) causes LOUD smashes while camper engine is cooling

[Brave Lady]

  Hi! I've been reading in here about vented gas caps, and I wonder if they would fix this problem I have.

I'll go drive the Brave somewhere, like today (because wiring melted and the brake lights wouldn't go off  ), and then I park again, resume my digital remote work and end up on the ceiling like an upside-down cat when the gas tank starts cooling off. There were two loud reports today that shook the camper.

This happens if I have anything less than half a tank of gas, which I have to guess at because the fuel gauge doesn't work. 

I love my Brave so much AND can anyone tell me if one of these vented caps might relieve the issue? I don't want the gas tank imploding, obviously.

Thanks very much,
Fuller

[RockwoodMike]

As soon as you stop wherever you are going, go to the gas cap and just open it and put it back on. Like if you are at a gas station..Take it off then right back on and see if that prevents the noise..If you have a cap that is not venting in air as the fuel is being drawn out by the engine, then when you open the cap it should release the vacuum 

[BigAlsVehicleEmporium]

My '72 Brave D20 came with a vented fuel system. The vented cap fits so losely that the vent itself could probably clog and I'd be fine Rockwood Mike's advice to open and close the fuel cap once when you stop is good. If that fixes it then yeah, you probably need a new vented cap.

[Brave Lady]

Thank you for the quick response, Rockwood Mike, removing the gas cap at each stop is certainly the simplest solution. But doing so while the engine is hot is a little sketchy, what with the hissing, fumes, and sometimes the cap blowing off across whatever space I'm in when it gets loose if I go too fast.

Do you think a vented cap would work on my Brave? I wonder if it would even make the fuel gauge work better, when that vacuum isn't developing while I drive?

Again, thanks for getting back.

[Brave Lady]

Big Al's, thanks!! I get it now. If taking off the cap releases the vacuum, then a vented cap will fix this issue. Awesome, I appreciate the further explanation. 

[Brave Lady]

Big Al's and RockwoodMike, would you fellas happen to know where I could get one of these fabled vented gas caps? Another post suggested a Gates 31705, but that appears to be the standard, nonvented cap I currently have.

Now that I know what I need, I can't find it!

[RockwoodMike]

https://www.amazon.com/Gates-31705-Fuel-Cap/dp/B000C2SCQ0

Here it is on Amazon..

Now on my gas tank that I am cleaning up and replacing the sending unit on it, it has a vent built into the top. It just has a hose (Not in the picture but you see where it would hook on circled in red)

That is a wide open vent..Easily letting in air when fuel is drawn away..
When you try to open the cap after running it, it would be interested to find out if you are letting off the vacuum..

[RockwoodMike]

Wait..That link is the non vented cap that you already have..Sorry about that..

[BigAlsVehicleEmporium]

Brave Lady,

I think the cap you need is Gates 31692 for the locking option or Gates 31648 for the non-locking version. This is the vented cap that went on Dodge trucks in the 60s and early 70s.

Hope it works!

Alan

[Oz]

I wouldn't worry about slowly removing the gas cap with the engine hot. And, it won't go flying since it's actually being vacuumed inward.

No, it shouldn't affect your gas gauge.  Fuel gauges becoming inaccurate is a pretty common issue considering the age of these rigs, due to either float or malfunctioning gauge.

[Oz]

I wouldn't worry about slowly removing the gas cap with the engine hot. And, it won't go flying since it's actually being vacuumed inward.

No, it shouldn't affect your gas gauge.  Fuel gauges becoming inaccurate is a pretty common issue considering the age of these rigs, due to either float or malfunctioning gauge.