How does the digital fuel gauge work?

Yesterday morning I drove my car approximately 3 miles and parked it with 4 bars left on the fuel gauge (just below half). I got into the car about 7 or 8 hours later and had 1 bar left with the gas icon showing on my tachometer. As I was driving a few miles to the nearest gas station, the "mileage left to empty" display on the tachometer was slowly going up and I was slowly gaining bars (made it back to 3 bars). What gives with all this confusion?

2011 MCSm


Thanks!

 

I belive it works on vacuumn. And if not then on the voltage the computer reads from your tps. (throttle position sensor ). So basically if you go and push the petal to the frloor from a dead stop and go up to 60 and check the miles to empty and your tripometer then just go easy on the car and accelerate easy, you will get more miles till empty. Hope that made sence

 

It's just not very accurate at all. The first few bars go away after about 100 miles or so, and the last few go away every 20-30 miles.

 

Have not heard of such a thing!! If I'm down to 4 Bars it stays there, I usually fill Molly when she is at 1/2 Tank anyway. I still don't think this should be happening.

 

hmmm. Mine doesnt change after being turned off but maybe a bar if on a hill...

 

+1

 

Really? My first bar drops at like 100 miles or so then drops at a pretty steady rate from there

 

It works like this...

You drive about 250-400 miles and then you put gas in. Somewhere between 1/2 and empty.

For me it works out to an average of 30 mile per gallon. (mostly shorter trips to the store) I got 34.5mpg on a highway trip...

 

I've had some funky readings, after filling up it says full, then 20 miles later it's dropped down by 4 bars (of average driving, nothing crazy) then after starting it up the next time, it's jumped back up to where it should be after such a short period.

Also, have had experience getting $25 of gas, and it dropping drastically to the point of 3 bars and a fuel warning, and then going back up after letting it sit shut down for a while.

Not sure if it's a faulty gauge, temperature, being on a hill, who knows. It's only been twice, but startled me none the less. Not leaking fuel or anything, I'd notice that.

 

How it works you ask?
In the tank there is a sending unit that has a float arm on a pivot. At the pivot there is wiper and a resistance coil wire which a voltage is applied. As fuel is used the pivot arm wiper moves down, the resistance is changed which changes the voltage. The voltage is measured by the computer and converted from an analog to a digital format, then displayed.
Here is what a sending unit looks like:
http://cgi.ebay.com/ebaymotors/MINI-...Q5fAccessories

The problem with most fuel systems is that the tank is not a perfect box shape due to the fact that the tank needs to be put in a confined location. In order to get as much fuel as possible in a confined space the tank must be make to fit the space available so it may be an odd shape.
This is why gas gauges in most cars tend to be non linear. Other factors that can introduce more error or sudden changes in the display is the incline of the car. Going down hill or uphill. Plus the baffles in the tank which keep the fuel from sloshing around can hold fuel back from the sender float area especially on an incline.

Also the sender does not take into consideration the filler tube so you may have 2 or 3 gallons above the full marks. This makes the gauge read full for a long time then drop off faster (non-linear).

Don't try to use the miles till empty readout and try to determine how much fuel is in the tank. MTE is calculated based on your current speed or fuel usage rate and will change as your driving changes.

 

Thanks for all these replies...makes sense.

More facts about the case...when I filled the tank at the gas station..it took 8.9 gallons. So despite reading 1 bar (but then going back up to 3) when I first got in...there was still about 4 gallons left in the tank. We have a 13 gallon tank right?

Thanks again.

 

Maybe a bad sending unit? I think on GM vehicles the sending unit will read past the full marker but that deff doesnt seem normal

 

For me I lose a "box" every 50km. At every fill up I go up to 550km. I think there are about 11-12 boxes so it should work out

 

I had this problem fixed on my 08 MCS. I found the gauge to be very accurate. Once I overlooked the gauge and discovered the gauge showing only one mile till empty. The reading went to zero as I was pulling into the gas station and the engine started to sputter. When the gauge broke (after 2 years), it was repaired by the dealer. The symptoms were exactly what you describe, the gauge went up and down. The dealer explained that there are multiple sensons in the tank and one of the sensors went bad. From what they described, the gauge is not a float.