Fuel Gage and Fill Considerations
I finally got around to poking through Toyota's TechInfo Web site today and found some interesting info about the Prius Fuel Gage and Tank/Bladder system: (Section 4 of Prius Technical Training document "Engine"):
Regarding Fuel Gage Inaccuracy and Filling Problems:
1. The fuel level sensor is direct-acting and involves a magnetic float which rides in a tube with a surrounding coil. A Level ECU then measures the slight difference in coil potiential.
2. There are also two sensors that sense side-to-side and front-to-back vechicle level and are used in calculating/adjusting Fuel Level. There is also a Temp Sensor in the tank.
3. There is supposed to be virtually no Vapor/Air space in the Fuel Bladder. The Bladder Expands and Contracts with changes in Fuel Fill and Fuel Temperature.
4. The Bladder cannot expand as much at lower temperatures. At 14F the tank capacity is reduced by more than a gallon.
5. The document states that filling the tank with the vehicle on an incline might result in the vehicle runnning out of gas with 3-4 Bars Left if the incline sensors are out of calibration.
6. Filling the tank REQUIRES that the GasPump nozzle seals into the rubber seal of the filler neck in order to inflate the bladder during fueling. Cycling the nozzle in&out while filling or having a poor seal due to nozzle damage will reduce the amount fuel accepted by the bladder.
I presume that this means that air has bled into the bladder ... burping the bladder at the end of the fill by cycling the nozzle slowly in&out, adding more fuel, etc. seems to purge the excess air and allows one to add as much as 1 gallon of fuel.
7. There is a NOTE that overfilling may cause EVAP system problems (fuel pushed into EVAP System)
8. They also note that the HV ECU tracks many pieces of data over 3-6 weeks of driving to refine MPG calculations (Driving patterns, mph, load, etc.)
So, this helps to explain many of the Fuel Gage and Fuel Fill inaccuracies and idiosyncracies.
My Fuelish Rules:
1. Fill tank on a level spot!
2. Inspect Pump Nozzle for scratches, dents and dings. Try to use a one in good condition.
3. Take you time in filling the tank. Slow filling seems to help in ensuring a full fill.
4. Try not to fill the tank in frigid weather (fill the tank right after leaving a heated garage).
Hope you've had your fill on this topic because my bladder is full!