Fuel gauge reads prematurely empty

There is no real fix for this except to change out the entire gas tank. I don't know how comfortable I would be to drive 150 miles when the empty light is blinking. I would probably just fill up at 200 miles every tank and call it a day. I wouldn't want to be stranded and run out of gas because my gut told me there should be more in the tank.

 

A couple things... The bladder in cold climates if never getting a full tank can have a hard time stretching back to normal size. This usually can be resolved by really long all day drives in super hot weather.

But I suspect that issue won't resolve the guess gauge, though when you're trying to use a float based fluid level meter inside something that acts a little like a water balloon, who knows, maybe that'd fix it?

If all else fails a used fuel tank assembly at the junk yard is cheap and comes self contained with all the components mostly inside, so it's a very simple swap with just bolts, fuel line, an electrical plug and fuel filler neck to deal with.

 

When u teach ur daughter that you're always there to bail her out she learns that you're always there to bail her out....

 

The fuel tank on a US market Gen 2 Prius is a sealed assembly. You cannot access anything like the fuel pump or the level sensor without cutting the steel shell apart.

I have done 4 tanks on different Gen 2's, and 3 of them were to correct fuel gauge problems.

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Noticed the same problem with 2 of my gen2s, after sitting for a few months. A few cycles of drive 100-150 miles and fill along with some slightly warmer weather has improved it. Now I keep the mileage per tank to 200 miles or less, in the hope that the volume of gas will keep the bladder stretched out. My latest issue is an avg mpg that is off by 5 mpg. New to me car, cleaned the MAF and put in a new air filter because it kept showing low 30s mileage. I did the calculation and am getting about 40 even though the readout for average is 35. Will calibrate the gage today just to see what happens.

 

@PriusCamper, I believe that driving an extra 150 miles after the gauge shows empty proves that the bladder isn't preventing the tank from filling. Instead, I appear to have an electrical issue that's causing the gauge to read empty when it still has around 6 gallons left. Since it seems that Toyota made the gas tank level sending unit non-replaceable, another fuel tank may be the easiest fix, or go by miles driven. This is my 3rd gen 2 prius. I've had to replace the bad display capacitor on all three, but this is my first one with a bad fuel gauge.

@MCCOHENS I'll calibrate the fuel guage on mine next weekend and see if that helps.

 

It is a common Problem. Mine shows empty with 3 gallons left. When I fill up I get about maybe 7 or 8 gallons. In hot weather it is better, cold weather it is worse. I usually reset my trip meter and drive 375 miles then fill up. Doing this I get about 9 gallons per fill up. I just treat the car as if it has a 8.5 gallon tank. LOL. I wouldn't spend the money for a new tank due to the cost and age of the car. Mine is a 2008.

 

Old age and shrinkage....happens over time....if you place 2 5 gallon gas cans next to each other you get a good visual of just how small that volume is..IMHO. The good thing is gas stations are all over and just change your mindset on your capacity. The electronic fuel guage is just an algorithm that the cars computer uses to calculate quantity. Like a range estimator in a Tesla that has a full charge but then only goes 182 miles to "empty" in -3 degrees temps. It just ....is.

 

That’s your best bet if you can pull it off.
Fill it slowly, turning the nozzle to 90 and 180 degrees.
Carry a 5 gallon can with a narrow nozzle that fits the inlet, and start driving.
You won’t be the first or last

 

I don't think that the bladder "shrinks" so much as it becomes less flexible. It's a short square box made of a plastic-resin with pleats around the outer edge. It expands or contracts vertically depending on the amount of fuel. (think of an accordion)

The white box has the pump and level sensor.

As I mentioned, I usually see problems with the fuel level sensor. Normally the sensor has low resistance (4 ohms) when full and high resistance (110 ohms) when empty.

When part of the sensor's potentiometer gets worn, it has high resistance or open circuit - anything from 200 ohms to mega ohms. All of those will display as empty (flashing pip) on the gauge.

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Weigh your scale at a landfill, with a full tank, then again when it's "empty"?

 

Interesting. You mention that the white box contains the pump and level sensor, and that a potentiometer on the level sensor wears, producing a high resistance with a presumably correspondingly low signal voltage. Is there a float in the white box with an arm acting on the potentiometer like a traditional gas level sensor? Also, it looks like your picture is with the tank upside down and the case split open, is that correct? If the potentiometer were the issue, then would it be feasible to split open the tank housing (as you have) and replace the worn level sensor? Potential issues could be sourcing a new level sensor (since toyota views it as a non-replaceable item) and putting the tank housing back together when the repair is done. What are your thoughts on this?

 

^ Took too long; can't edit original.

 

I really had to weigh in when I read this. My Gen 2 Prius does exactly the same thing also right after replacing a capacitor in the speedo. I don’t think that’s a coincidence; the only difference is mine reads empty at 3/4 tank. I just keep track of miles and stay on the highly conservative side of potential range.

 

I sliced the old fuel tank open because I was curious about what was inside. That tank assembly is "right side up".

In theory one could remove the tank, cut open the steel shell and the plastic pump & sensor housing to replace the sensor (assuming you can find something compatible).

Then you would have to seal the pump housing and the shell with zero leaks from either - or the MIL will stay on with evap system codes.

Oh, and you want to be sure after repairs that the tank won't rupture in a crash.

It's "possible", but REALLY not practical.

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You think the capacitor repair is affecting the fuel gauge?