Why the top fuel level bar lasts so long

[CTScott]

There have been quite a few threads in the past that discussed this, but here's a Mythbusters style analysis of it (fortunately, minus the large explosion at the end):

Today I am working on the range until empty and fuel level in gallons (or liters) features for my YarGauge project. I drained Crashy's fuel tank and then added 1 gallon (precisely measured with a one gallon laboratory flask) at a time while recording the data values from the analog to digital converter, which I have connected to the fuel level input to the instrument cluster.

I plotted a graph to see how linear (or not) the fuel sender value is in regards to the volume in the tank. From bone dry to the top of the filler pipe, I was able to put in 12.8 gallons. The ADC value stops changing at just over 11 gallons.

So, you can put just shy of two gallons more into the tank than the fuel gauge can measure.

[b_hickman11]

Originally Posted by CTScott

There have been quite a few threads in the past that discussed this, but here's a Mythbusters style analysis of it (fortunately, minus the large explosion at the end):

Today I am working on the range until empty and fuel level in gallons (or liters) features for my YarGauge project. I drained Crashy's fuel tank and then added 1 gallon (precisely measured with a one gallon laboratory flask) at a time while recording the data values from the analog to digital converter, which I have connected to the fuel level input to the instrument cluster.

I plotted a graph to see how linear (or not) the fuel sender value is in regards to the volume in the tank. From bone dry to the top of the filler pipe, I was able to put in 12.8 gallons. The ADC value stops changing at just over 11 gallons.

So, you can put just shy of two gallons more into the tank than the fuel gauge can measure.

Attachment 31330

This is not only on the Yaris. Every car or truck I have owned has the same "problem" as the Yaris. It's the norm.

[CTScott]

Originally Posted by b_hickman11

This is not only on the Yaris. Every car or truck I have owned has the same "problem" as the Yaris. It's the norm.

Exactly. Because the level sender is in the tank and the fill pipe is above, this will always be the case. I just thought it was interesting to see the data on where the real cut off is.

[DerFlosser]

Cool Scott! Confirmation is always a good thing.

[BailOut]

That's a great data set, CTScott. Thanks for sharing it.

[Kal-El]

Thanks for this project scott! It's nice knowing what the actual capacity of the tank is since we never fill up from bone dry to find out. 12.8 gallons vs the 11.1 capacity that is the official spec. So approximately 1.7 gallons over the gauges "full" level. Certainly explains the extra 50 or so miles we get out of the first bar!

[ddongbap]

+1. CTScot for president of something.

[jh3cuemd]

nice investigation CTScott, pro-like!

[tk-421]

Thanks for the great info as always, Scott!

Originally Posted by CTScott

here's a Mythbusters style analysis of it (fortunately, minus the large explosion at the end):

You have to admit it would've been awesome to see though...

Do you know how each fuel bar correlates to those graph values? It would be nice to see when the fuel meter comes goes down from 3 to 2 bars, for example.

[41magmag41]

Thank you Mr. Scott. Always wondered why the first few gallons went so far. Nice to know we can stuff almost 13 gallons in the tank.

[CTScott]

Originally Posted by tk-421

Thanks for the great info as always, Scott!



You have to admit it would've been awesome to see though...

Do you know how each fuel bar correlates to those graph values? It would be nice to see when the fuel meter comes goes down from 3 to 2 bars, for example.

It's pretty gray as far as the bars go. They're bands are quite wide and the cluster's computer takes quite a while to decide that it's OK to update the level.

I understand why, as driving around and watching the real-time raw data, the fuel sloshes like crazy and the level bounces over a range of 2-4 gallons.

[Yaris Hilton]

The gauge has to be averaging the level over a fairly long period of time.

Yesterday I parked mine in the garage and shut it off with the second bar showing on the gauge. When I started it up again a few hours later, it came on with the last bar blinking. Never saw it do that before. I figure it had just reached the threshold for changing bars, but it hadn't been there long enough before I switched it off.

[TheSilkySmooth]

Thanks scott! I have been checking my fuel mileage the old fashioned way, and filling the tank pretty full (to second pump clickoff on slow fill) was wondering how well the evap system can deal with an overfull tank w/o flooding the canister or the purge lines and causing trouble.

[CTScott]

Originally Posted by TheSilkySmooth

Thanks scott! I have been checking my fuel mileage the old fashioned way, and filling the tank pretty full (to second pump clickoff on slow fill) was wondering how well the evap system can deal with an overfull tank w/o flooding the canister or the purge lines and causing trouble.

The evap vacuum line connects at the top of the filler pipe, so theoretically it will only get wet when you overflow the filler pipe. Although, if the filler pipe is very full (but not overflowing) it is quite possible for the fuel to slosh around enough to get some where it shouldn't be.

[kameele]

CT--there's no end to what Crashy can do!! it's nice to know that there is a little more capacity than the spec says. i know nothing of the structure of the gas tank. is there any heel left when the level hits the gas line? That is, any un-useable gas left behind?

[CTScott]

Originally Posted by kameele

CT--there's no end to what Crashy can do!! it's nice to know that there is a little more capacity than the spec says. i know nothing of the structure of the gas tank. is there any heel left when the level hits the gas line? That is, any un-useable gas left behind?

For the test, I started with the tank less than half full. I drained it by disconnecting the filler pipe from the tank and using a hand pump to siphon the gas. I drained it until the pump was sucking air and the bottom of the tank had less than 1/4". I started the car to see if it would start and run for a bit and after about 5 minutes I got bored and shut it off and started grabbing my data. The bottom of the tank is flat with a couple of ridges for strength, so I think it still running was due to the well within the sender unit.

Here is what the bottom of the tank looks like. It is basically flat with a small recess where the sender unit sits.


Here is a view of the sender unit, looking down into its well where the fuel pump sits. You can see the circle facing the float which is the intake.

[kameele]

So basically, if you run out of gas, the tank is dry.

[CTScott]

Originally Posted by kameele

So basically, if you run out of gas, the tank is dry.

Very close to dry. When it stops running the bottom of the tank is barely covered.