I took the vent tube and the fill hose off and there are no blockages; can you help?
Been “topping off”? If so, some systems will allow gas to get into the charcoal canister through the vent line and saturate the charcoal bed. Since the tank can’t breath out through a saturated bed, the hose handle thinks the tank is full and shuts off.
Maybe it IS full and it’s your fuel gauge that is not reading correctly…
Excellent point, Caddyman. Perhaps the float is hanging up.
Erroneous fuel gauge would be my guess too.
I don’t think an 81 Chevy would have a charcoal canister, mine didn’t.
Pull the nozzle out a bit next time (but still leave it in the filler tube,) and see if that helps get more in there. If it spills out, then it’s likely full if you don’t have a blockage. GM fuel gauges, especially older ones, aren’t the most reliable things out there.
Since 1971 all vehicles sold in the U.S have been required to have fully sealed fuel systems that prevented any hydrocarbon molecules from escaping to the outside air. It was that year that charcoal canisters were implemented. Every vehicle from 1971 on has one. The charcoal canister has been the only device used to trap hydrocarbon molecules. In the early years they had them under the hood, but now they all seem to be under the vehicles.
The systems have evolved over the years, and since 1996 all vehicles have been required to monitor the system’s performance (via the OBDII system), but the canisters go back to '71.
Passenger cars did not get ORVR (Onboard Refueling Vapor Recovery) until 2006…Before that, when the fuel cap is removed, the system is open…They still had anti-spill roll-over protection valves before that, but the OP has removed his filler neck and tube and found no obstructions…
The charcoal canister allows the tank to breath when the cap is on without allowing hydrocarbons to escape. It predates the ORVR requirement.
The Refueling Vapor Recovery is a different system, requiring vapors from the tank to be recovered during refueling. The two, while interrellated, are not the same. The requirement to prevent hydrocarbons from escaping with the cap on, a “'sealed system” goes back to 1971, My '72 Vega had a charcoal canister.
My vote would be towards an erroneous gas reading and the tank actually having more in it than thought.
One exception might be if the tank was crimped in due to vacuum pull. A visual inspection should detect something like this with the tank straps not mating completely against the tank.
While I’ve never personally seen a GM tank in this condition I have seen a few VW tanks that suffered this problem with the amount of gasoline required to fill the tank (from empty) varying between 3 to 6 or so gallons.
Engine vacuum would suck them down like inhalation on a paper bag.