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I have a persistent airlock when filling the gas tank of my '51 Club Sedan. Initially thought it was a hole in the gas pickup tube because every time I filled at a gas station the tank would only take about 5 gallons before overflowing up the filler tube. Fact was that my gas tank had never been able to be completely filled since I got the car running. It's been an air lock happening every time I refill.

I have avoided the problem only once so far! That was by using a jerry can and pouring in petrol very slowly, but other times even this didn't work. That time I had disconnected the breather rubber tube right down near the tank, and I could blow or suck air through that quite easily.

So the breather appears to be open.

Has anyone had the same problem, and have a solution for it?

Maybe a bit of an unconventional way of doing it (and it won't fix the problem )but have you tried driving the rear tire on a block of wood/ brick / lawn mower ramp when you fill it up? It would make it easier for the air to get out while filling up.

The symptom perfectly matches having a clogged whistle in the tank vent line, assuming your car has the whistle.

As O_D mentioned the whistle, even if it was not completely present and functional if the tank was ever treated and one of the rust encapsulator products was sloshed around inside the tank, that area could well be coated with the stuff and the vent blocked. I know that stuff blocks the pickup tube if tank is allowed to dry in a position where it collects below the tube opening so it could have done the same at the vent if the tank was allowed to dry upside down.

I had the same problem after I had the tank on my 48 boiled out and coated. There is metal disk that seals the seal the vent-it was sealed to the top and blocked the vent. I solved the problem by drilling a 3/16 th hole through the top of the round vent housing and the disk. I then soldered a Penney over the cleaned metal with a soldering iron. "Important to not use open flame" .

While getting to know my '47, I had a similar problem. The tank would overflow(as if full) when the pump nozzle was pumping at full and fully inserted into the filler neck. I discovered that it fills nicely when I pull the pump nozzle out a few inches or perhaps half way, and pump at about half speed or less. With that method I get a nice whistle from the vent-alarm and no burps from the filler neck.
Might be worth a try.
Good luck,
Mark.

Another possibility is the vent tube being of too great a length. It shouldn't have any dips or bends to create anything similar to a plumbing p trap. If it is bent to create one, pull the tube and/or trim it a little shorter to prevent the vapor from liquifying into the tube preventing proper ventilation.

Hi Lee,

Sounds like these guys are right on to it. When you wet your whistle, the breather gives out. Looks like the tank has to come out.

I wish my '52 had a whistle!

BTW, I stumbled across the whistle on page 19 of the 1949 Owners Manual herehttps://www.packardinfo.com/xoops/html/downloads/49_OwnersManual.pdf

Cheers,
Jeff

Thanks to all for your replies.

My tank was in very good condition, I could see a shiny original surface inside through the sender opening after I did basic cleaning. It hasn't been treated / coated with anything. I suppose the whistle could be clogged to some slight extent but as mentioned, I have blown air by mouth through it (into tank) causing the gas to start rising in the filler tube. Have also sucked air from the tank through the whistle and there didn't seen to be much resistance then to air flow in either direction.

Got me wondering if perhaps there was a blockage there and I have now cleared it by blowing and sucking through the breather tube.

Looks like it will be necessary to try filling it again - carefully - and using suggested method(s). So I'll do that after the next drive. The point about height of the breather outlet is pertinent . . . the pressure differential will increase with height, so I guess a small / minor blockage would cause enough back pressure to back up the inflowing gas and make it overflow. I may yet try a (gentle) blast of compressed air through the whistle to see if that helps!

Will post my findings soon.

I have just developed this same issue this summer, it takes about 15 minuets to fill my gas tank on my '55. Everything sould be clean & clear....most infuriating!!