The Stovebolt Site Forums The Shop Area General Truck Talk Gas tank ventilation question for 1947 Chevy 1-ton 3800

Hello,

Does somebody know something about the gas tank ventilation on a 1947 1 ton pickup truck? The gas tank is located under the bed (this is the right/ original position for the 1 ton). But I don´t know if I have the right filler neck (maybe there´s a ventilation on it, but I don´t know). I'm generally missing a ventilation hole.

Maybe I can find somebody here who have a picture for me with the original gas tank filler neck and ventilation hole.

Thank you!

Sven, Perhaps this snapshot from the 1947 Shop Manual showing the 1, 1-1/2 & 2 Ton fuel tank diagram will give you what you need ??? The ventilation should be through a vented gas cap. Those often get debris in them which cause the vent to stop functioning properly. Probably best bet will be to order a new vented cap. Others should be along to offer their wisdom.

Thank you Dan!

That´s right. The snapshot shows exactly my fuel gas tank. It seems, previous owner changed the cap to a lockable cap with key and without ventilation. Next week I will take out the reservoir from truck to check inside for rust. Then I will decide if I add a ventilation or if I will change the gas cap.

Sven, Glad it was helpful. A proper vented gas cap will make sure the fuel flows the way it should. Trying to add a vent would be much more costly than getting the correctly vented cap. Let us know how things turn out on the internal check for rust.

Project postponed. No time for the moment to check the gas tank for rust. Maybe in summer when I pull out the engine for clutch replacement and gearbox check/ overhauling.

This post made me get off my backside and check my spare parts box inventory to see if I have an extra vented cap......nope! Time to order one. My current cap is pretty darn old and it would be so nice knowing there's one out in the box in the garage.

When you receive a new aftermarket gas cap, do not assume that it is vented. I haven't bought one over the past 15 years from our normal sources that were indeed vented as the originals. They all have plastic parts in them which does something I'm sure, but venting is not one of those things. When I get one of those, I carefully drill a 1/8th inch hole in the plastic disk viewable in the underside of the cap.
No need to drill past that disk. That is where "carefully drill" comes into play.

Show a picture of your cap and the drilled hole.

This is te cap that's been on my truck for several years. Worked fine until all the gas tank nonsense and now causes a vacuum to be created in tank.

I have an AC Delco GT-55 . No plastic parts. Vented.
Picture of box. Sorry I can't find a picture of the cap.

That cap looks different than the ones I have purchased. The plastic in mine is blue without holes.
The ones I had trouble with also caused a vacuum in the tank. I would hear a giant sucking sound when I removed the cap, especially when the fuel level was low. I do not know if they allowed excess fumes to escape. In my opinion, they are supposed to vent, and not retain a vacuum.

That's correct. The one above worked for a few years then after the bad gas and broken Spectra tank fiasco, it ceased venting and created a giant suction like you describe.

I'll try drilling through that plastic and see what happens.

I took the cap apart by releasing those two tabs holding that metal retainer ring in place. There's a rubber valve with a spring inside the plastic thing. I removed that and reinstalled the cap. It appears to be venting properly now. We'll see what happens.

I’m just guessing that was designed to only release fumes when pressure is enough to open it, so removing it will likely result in fumes in your garage constantly.

DES57, Yes, that's the way most vented caps work, with pressure change to either let air in or fumes out. But with a small enough hole fumes wouldn't be a big issue in the garage, IMO. It only vents fumes as the contents inside warm up.
I once collapsed the tank on a boat when I forgot to loosen the manual vent screw on the cap. Running along just fine and heard a "BONG". Looked back and the tank had a big kink along one side. I don't know if the fuel pump on a 47 could generate that much suction or if it would starve the engine for fuel before the tank collapsed. But it's important to have a proper vent.

Yes, there was a little valve in there I suppose to allow the suction to draw air from the outside and then pop open if the pressure inside got too great. However in this case the little valve was not opening and the fuel pump was creating a huge vacuum in the tank. Now it's just permanently vented we'll see how it works so far so good.

How small is the hole where that little valve is. If it is around 1/8" or less, you should be fine, just like the original vented cap.

It's a little larger than 1/8".