Fuel Gauge Errror

My Prius starting have refuel issues - the dreaded constant cut off as I added fuel. The root cause, it turned out, was the anti-overflow check valve located up near the fueling port (in the driver's side wheel well, under a plastic cover.) I removed that check valve assembly and took out the bad check valve (but returned the valve's plastic, now empty case, to maintain the hose connections.) That problem was solved.

However, at that point, my tank would only accept 2.3 gal of fuel (from blinking 'empty light' to Full.) Did not believe the fuel gauge (through stranger things happen with these bladder tanks, so never sure which was the real issue - sensor or tank.) So I decided to empty the tank when the fuel light blinked empty again (after driving 15 more miles to be certain.)

I drained 5 gals. This proved the gauge was terribly wrong (yes, two gals proved it but I was determined to make 110% certain.) So I then I drove to the nearest station (figured there was still fuel in the tank and think there was over a gallon) but did bring a gal of fuel with me in case.

The car now took 9 gallons exactly and the fuel gauge shows full (strange if it didn't.) So close to the 10+ gal the tank should hold. Of course, no issue filling the tank to full now that the 'overfill check valve' was removed. Don't want to think what that replacement item would cost from Toyota. Of course, I will never try to add further fuel after the nozzle cuts off since that fail safe is disabled.

Will now track milage to see if the gauge follows the tank (and especially for the next fill up.) So I set the ODO A to zero to keep track. I'll see if that issue repeats or this fixed the fuel sensor so it tracks properly now.

Aside: To drain the tank (partly or all) first remove the wipers, motor and lower tray to gain access to the fuel line. Disconnect that hose from the fuel injector pipe. That rubber hose can stretch to easily reached a 1 gal container set on top of the engine. Put the container in place. Then locate the connector that powers the fuel pump relay. That is in the back mounted on the hybrid fan. Use the black wire on the connector and connect that to the plus 12 volt battery nearby (a two foot thin wire will do). I then ran the pump and filled the container. Disconnected the jumper, put that 1 gal of fuel in a 5 gal container, and repeated till I got 5 gals. That proved the gauge was certainly lying.

One could have run the two to three hundred miles over a week or more while that low fuel light blinked to empty the tank. That for me would be far too unnerving and never knowing where it will really cut out could prove dangerous. So I did the safer and much faster way. This only took an hour and half of easy work (including reassembly.) I will use that 5 gal of fuel for the lawn mower.