Tank pretends it's full

When I recently put more biodiesel into the tank with an electrical pump, some of the biodiesel spilled out. I put the nozzle further inside, and the problem went away. Soon after, I had those fuel starvation problems, which the fuel filter fixed. I don't know if the two events are related or not.

Recently I tried putting more fuel into the tank with my own hand crank and the fuel spilled out again. The tank was about one-eighth full at the time. I tried forcing it in, and also dripping it in slowly. It kept spilling over after about 8 ounces was put in. I went to a gas station to test the electrical pump, but it just clicked as if the tank was full. I brought it to Terry's Automotive, but they said it could be many things causing this, and that I should go to Best Motor Works, which is the place that put in the tank; they'd know what kinds of valves and fuel lines are involved. I dreaded having to drive all the way across town, with very little fuel left. I figured out, though, that if I slowly put in about one cup of fuel at a time and waited a few minutes, that the tank did actually take in the fuel. A quick calculation told me that a gallon would require 16 cups, and each cup needed about 5 minutes to fully drop in, so one gallon would require about 80 minutes. A daunting task, but worth the security of having a more than a few gallons in the tank. Each gallon should get me about 40 miles and a stress-free ride.

Sometimes the fuel went in quietly, sometimes it gurgled — clues to what is happening inside. After putting in about 10 cups I heard a gurgling and a mechanical "kachink" sound. This was exciting, in that it could be some kind of valve popping open, or some obstruction releasing. I put in the cups in little more quickly, and there was no spillage. Then I tried putting in fuel directly from the nozzle (which puts in about one cup for every two cranks, a gallon takes less than a minute, so there is a lot more fuel going in at once).  Again, no spillage. This was very encouraging and it meant that not only did I have enough fuel for a longer trip, but I wouldn't have to take the trip to the mechanic at all.

I take away the lesson that I shouldn't let the tank get down to less than a quarter full, even though I don't know if that was the problem here. My sense is that some valve system in the tank needed to be surrounded by fuel in order to open up properly, or maybe a certain maximum of air inside the tank had to be avoided, in order for the tank to be able to receive fuel. Another speculation is that the biofuel's sticky, gelly deposits caused the valve to stick and that enough fuel dissolved the sticking. Who knows?

I had heard that one shouldn't fill the tank up all the way, and also never let it get all the way empty, but not that a quarter tank is already a problem.