Fuel Gauge calibration

wow, that was a great run!

are you sure you lose the gas gauge when the 12v is down? i have never heard that before.
how many miles on her?

 

yeah, that would be quite a coincidence.

 

Having just purchased a 2007 Toyota Prius I have encountered the bizarre issue of the notoriously inaccurate fuel gauge – after filling the fuel tank the first “pip” didn’t disappear until the vehicle had travelled 180 miles!

In its later versions of this car Toyota has abandoned the Fuel bladder in favour of a conventional resin/plastic fuel tank with a normal float system of measuring the fuel remaining; the fuel bladder being employed in aircraft and racing cars to avoid the dead space above the fuel where fuel vapour can accumulate and therefore exhaust to the exterior causing atmospheric pollution and increase the risk of explosion in the event of an accident!

So I conclude that the only way in which the remaining fuel can be measured is by the on-board computer! The fuel is injected rather than vaporised as in a carburettor and therefore the quantity of fuel used per second can be measured; the odometer tells the Engine Management Unit [EMU] the distance travelled per second and this enables the computer to calculate the MPG continuously shown on the display second by second.

So the quantity of fuel used shown by the 10 “pips” on the fuel gauge is actually not measured directly but computed, and this is why this measurement can be put out of kilter if say the battery is disconnected without maintaining a power supply and the settings for the fuel levels are lost. This is why the cure to reset the fuel gauge is a complicated electronic manoeuvre using the odometer Trip A reading and the ignition on/off buttons with the vehicle stationary on level ground.

 

Since writing this piece I have found a comprehensive article about the construction and operation of this unusual type of fuel tank. My assumption that it would not have a float mechanism was wrong. There is a separate mini-tank alongside the main bladder tank which operates much as the water level indicator on a kettle. This conveys the information to the fuel gauge in the conventional way by electronic signal. Unfortunately the design leaves it prone to inaccurate interpretation of the level of the fuel in the main bladder tank if for instance it is refuelled with the car on a steep slope and this is not always easy to correct afterwards - the article in question is entitled

 

I have a 2008 Prius with 117,000 miles. Everything worked fine until last September when the gauge would show empty after only 200 miles or so . . . I went to my (no longer) trusted local dealer, who charged me some nice change to recalibrate the gauge, only to find out that they had done nothing. They had a second try only to make it worst so that it would start showing empty at 150 miles!

I used to be able to use at least 8-9 gallons of the tank on a regular basis but now we regularly refill with 4-6 gallons at best as my wife is concerned about running out of gas. We regularly monitor MPG vs miles driven so I guess we could drive it until we know we have used 10+ gallons but we are concerned about the possibility that the bladder system fuel "tank" has deformed and we do not want to run out of gas.

My local Toyota dealer has been very defensive about it. Actually, last time I was there I was invited to talk with a team of managers and I was given a very formal speech being told how they did everything by the (Toyota) book and that if I wanted it really fixed, they could replace the tank for a couple of thousand dollars or so.

I used to swear by Toyota products but not anymore, especially with the way I was treated. I would have been OK if they told me up front they could not fix it but NOT when I had to pay for it and they acted like nothing happened!

 

My mechanic says to try pulling the fuse on your dome light for a few seconds, then plug it back in. this only works on the gen2...

 

I just got a 2009 Prius. I replaced the capacitor in the speedometer cluster, but now the fuel gauge needs calibration. Will this happen every time the 12V battery is disconnected? And why is there no button for such an essential feature? I will attempt the chicken dance described above, but Ive already spent a lot of time "dancing" with my SKS key fobs, unsuccessfully, that I am tempted to just use the odometer.

 

Kinda, yes and no. Let me explain.

The fuel gauge will lose its knowledge of the level of the fuel tank each time the 12 V supply is disconnected. However, it self-recalibrates once the power is reconnected. How long this takes can be from 5 mins to 5 days. There is no need to do the "fuel gauge calibration" procedure, which in my opinion is a bit of a misnomer, what that calibration does is set the system to the 'horizontal-level' (or zero point) position. This procedure obviously needs to be carried out with the car on a level surface.

However, you may need to do the level zero-point calibration just once after replacing the combination meter (CM) (the speedometer cluster as you called it).

 

How often does the average person disconnect 12V power from their car?

I have never done any "recalibration" on my Gen2 or any others. Just hook up the battery and drive the car. Eventually the fuel gauge reads properly.
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Do you happen to know which fuse is for the Dome light? I'm assuming the fuse is located in the fuse box?

 

Saying "the" fuse box doesn't reveal which fuse box you have assumed it is in.

It's in the one under the hood. It's (at least in the 2006 book) a 15 amp fuse labeled DOME. (It also supplies some other things that have always-on power.)