My car now takes about 48 litres of fuel. I noticed though that the gauge was not going down. Given it is not a car that sips petrol I found this hard to believe. Fuel level sensing is done by the fuel tank sender. It has a float on an arm that moves up and down with the fuel level and that in turn moves a variable resistor.
# First Hypothesis - Gauge is wrong
I fitted a spiyda Gauge Wizard when I fitted the tank as the sender and the gauge were definitely not a matched pair. Because this needs calibrating, I had it accessible in the footwell so all the wiring (+12v, ground, sender, gauge) were all usefully accessible. I removed the Gauge Wizard and connected a multimeter between the sender and ground. I then drained the tank and pumped in 10 litres whilst watching the MM. The readings did not change as they should have done. The gauge is not a suspect.
# Second Hypothesis - Sender is wrong
Ugh, this means taking the fuel tank out. See the other posts about that and then 'disassembly is the reverse of assembly'. It took a while to get the car balanced right, but eventually I got it out, and removed the sender for further testing.
Connecting a MM to it and wiggling the float caused the MM to change as it should, but it felt 'crunchy' and not smooth. There is a hole in the case so I gave it an electrical contact cleaner enema.
I then plugged it back into the harness
and attached the MM into the car wiring
Waggling the float gave readings as expected.
Next step was to put it back in the tank and put some fuel in and see what it did. I made a horrble bodge of an unused fuel pump, a knackered car battery and some hose.
Running fuel in and out showed that the sender was still sticky. The value on the MM would only change if I gave the tank a smack with a hammer near the sender.
The sender is knackered. Bum.