Weird Issue with Gas Gauge

My 2010 did this while moving. I attributed it to sloshing in the tank. But the car was never 'on' while parked long enough to observe this, and I wouldn't have watched the 'guess' gauge while parked anyway.

My 2012 is still new to me, so I've seen the gauge spontaneously rise only 1 pip so far. My past non-hybrids all saw the fuel needle climb at times while the fuel sloshed around in the tank.

I see no reason for concern. PC has many threads about people noticing various 'guess' gauge anomalies. The reality is they are expecting far too much accuracy, linearity, and consistency from a cheap uncalibrated indicator. While many of us would dearly love to have accurate gauging, the automotive industry refuses to put that kind of money into it. Probably because most customers are not willing to actually pay for it.

Your 'guess' gauge -- the bladder in the 2004-2009 U.S. Prii makes these inconsistencies worse than on other cars -- really just tells you whether the fuel is high or low, and usually has plenty of safety margin when it reminds you to refuel long before you run out. Be thankful for that margin. I found out the hard way that one long-ago car had no margin, actually a negative margin.

PS. I'm going to guess that when your engine cycled on and off, so did the fuel pump, slightly changing the air pressure inside the tank and causing the bladder to flex. If the fuel level was close to the threshold between pips, it could very easily revert across that transition point. Even absent any movement, measurement theory says that measurement noise alone can cause one pip to randomly come and go.

As for changing 2 pips, not just one, remember that the sensing system is low cost and not very linear. On my car, that 9th pip is typically very short, representing less than half the fuel of the others. So it doesn't take much slosh or noise or other interference to go from 8 to 10 pips.

 

I think that fuzzy1 did an excellent job of identifying a possible cause of the pip count
change in displayed fuel level.

But neither he, nor you, nor I can be certain due the mind boggling complexty of the Gen II
Fuel and Evap system.

The fuel level displayed by the "guess gauge" is made up of at least four inputs: the measured
fuel in the bladder; "ambient temp," apparently measured in the air space between the hard
tank and the bladder; and inputs from the ECM and Body ECU.( I can't figure out what
these latter are/do.) Outputs from fore/aft and side-to-side inclinometers also have a
contribution.

Remembering the ambient temp sensor, another possible cause for the pip changes might be
that as fuel is drawn from the bladder, the outside air drawn into the tank bladder space
changes the ambient temp sensor signal, which changes the pip count. As the car sits the
ambient air temp would go back to the former temp… and a repeating cycle.

Whether the real cause is fuzzy1's or mine, or both, or neither, I don't see any real problem.
You've just found yet another of the many oddities that are part and parcel of the Prius/HSD
package.

If you care to try to puzzle this out for yourself, go here for the whole lurid story of the Gen II
Fuel and Evap System.

 

If gas gauges were truly accurate it would add hundreds, possibly thousands, to the price of a car.