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I realize that if you have driven for any time in the M5 you will probably notice that the fuel gauge is not exactly reading the amount of fuel in the tank. No, it is reading the approximate miles left before you run out of fuel and then translating that back into how much fuel there SHOULD be in the tank.

How to prove this theory. When you get down to below 1/4 tank, look very closely where the fuel gauge is sitting when you turn the car off. The next time you get in the car it is very likely that the gauge no longer says you have as much gas as you thought you had when you last drove it. Why? My guess is it is coming directly off the computer from the MPG that you have averaged since the last fill up and not the amount of fuel in the tank.

It would have been nice to have had an independent reading on the level in the tank and not an estimate based on MPG. I can always go look at that on one of the screens.

Ok, I am done with my nitpicking for today.

Mark
 

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I dont know if thats correct or not- but I have the same experience. My rule of thumb for any car is to keep about 100miles in the tank- so I just watch the range readout anyway. With all the craziness going on- I do not think you want to be at the mercy of the gas station working to have a travel distance of about 100 miles. JMHO.
 

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MAH said:
I realize that if you have driven for any time in the M5 you will probably notice that the fuel gauge is not exactly reading the amount of fuel in the tank. No, it is reading the approximate miles left before you run out of fuel and then translating that back into how much fuel there SHOULD be in the tank.
.........

Mark
My ABS sensor (left rear) failed last week, and this is the same sensor gives speed information.

If there should be any average calculation, my fuel gauge should have given the wrong values. It worked fine, to fine actually:

Even a small turn made the needle move 1/4 of a tank, and back again after the turn. So there is some electronic calculations, but those I believe is only based on if the car is moving or not, and it seems it's only a latency on the measurement. From what I know, there is only one sensor in one of the two fuel tanks.

Difference in measurements could also be caused by fuel returning from the fuel pipes/pumps/filters/regulator back into the tank, or that the car is not 100% leveled when park.....and so on.

I've made the same observations, but haven't come to the same conclusion as you have :)

Maybe DavidS could shed some light if he sees this thread.
 

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MAH said:
I realize that if you have driven for any time in the M5 you will probably notice that the fuel gauge is not exactly reading the amount of fuel in the tank. No, it is reading the approximate miles left before you run out of fuel and then translating that back into how much fuel there SHOULD be in the tank.

How to prove this theory. When you get down to below 1/4 tank, look very closely where the fuel gauge is sitting when you turn the car off. The next time you get in the car it is very likely that the gauge no longer says you have as much gas as you thought you had when you last drove it. Why? My guess is it is coming directly off the computer from the MPG that you have averaged since the last fill up and not the amount of fuel in the tank.

It would have been nice to have had an independent reading on the level in the tank and not an estimate based on MPG. I can always go look at that on one of the screens.

Ok, I am done with my nitpicking for today.

Mark
I am not aware of that analysis. I do agree that the gauge is always lower than when you shut the car off. I think it has something to do with the two sets of values you see when you unlock the OBC. One number declines while the other increases, but always totalling about the same amount. Not sure why BMW did this.
If the gauge only measured its reading based on mpg, how would it change when you fill up, since the car hasn't moved? It must have some way to measure the amount of fuel in the tank.
Wouldn't it be easier to measure actual fuel than run a sophiticated program to estimate the amount based on other parameters? Like trying to make a left turn by going right 3 times: do-able, but why bother??
I have run my gauge down to the bottom twice just to see how accurate it is. It is a little pessimistic ( a good thing for a fuel gauge), but not by much.
If someone has the ETK, maybe we find whether there is a float or measuring device in the tank..
Regards,
Jerry
 

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if you unlock the obc and go thru the tests, you'll find one that gives you your fuel load in liters, divided up, as i think we have 2 tanks for weight distribution. must be somthing measuring how many liters in each tank!
Mike
 

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My observation also is pretty much inline with yours Robert.
robertdn said:
My ABS sensor (left rear) failed last week, and this is the same sensor gives speed information.

If there should be any average calculation, my fuel gauge should have given the wrong values. It worked fine, to fine actually:

Even a small turn made the needle move 1/4 of a tank, and back again after the turn. So there is some electronic calculations, but those I believe is only based on if the car is moving or not, and it seems it's only a latency on the measurement. From what I know, there is only one sensor in one of the two fuel tanks.

Difference in measurements could also be caused by fuel returning from the fuel pipes/pumps/filters/regulator back into the tank, or that the car is not 100% leveled when park.....and so on.

I've made the same observations, but haven't come to the same conclusion as you have :)

Maybe DavidS could shed some light if he sees this thread.
 

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Not sure if the M5 is like the M3, but the M3 had 2 tanks, with pumps on either side to pump fuel between them to keep the two tanks equalized. If you ever ran a 3 series dry, you will most likely damage one of the pumps and/or sensors, or possibly all of them. This causes the OBC and the guage to read incorrectly.
 

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Just like HDclown says, on the M5 there are two pumps, on to move fuel from the left tank to the right tank, and one to pressurize the fuel and move it from the right tank to the engine.

There are also two floats to measure the volume in each tank. The sum of the readout from those two sensors is what is displayed on the fuel gauge.

The numbber is dependent on how the car is tilted when parked, on acceleration, braking, cornering etc.

There is also a lot of smoothing done to keep the needle from moving randomly when doing such manouvers. This smoothing is the reason it takes a while for the needle to stabilize after startup.

David
 

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DavidS said:
Just like HDclown says, on the M5 there are two pumps, on to move fuel from the left tank to the right tank, and one to pressurize the fuel and move it from the right tank to the engine.

There are also two floats to measure the volume in each tank. The sum of the readout from those two sensors is what is displayed on the fuel gauge.

The numbber is dependent on how the car is tilted when parked, on acceleration, braking, cornering etc.

There is also a lot of smoothing done to keep the needle from moving randomly when doing such manouvers. This smoothing is the reason it takes a while for the needle to stabilize after startup.

David
Good information. On a related note, I know we can adjust the OBC regarding fuel consumption by adjusting the "factor" which is defaulted to 1000. Is there any adjustment as to when the yellow light for "low fuel/reserve" appears?
Regards,
Jerry
 

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Had that problem with my old 540, one of the sensors went bad and I was getting a false readings on my guage and miles left until empty. :sad1:
 

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gsfent said:
Good information. On a related note, I know we can adjust the OBC regarding fuel consumption by adjusting the "factor" which is defaulted to 1000. Is there any adjustment as to when the yellow light for "low fuel/reserve" appears?
Regards,
Jerry
Jerry

I believe the yellow light for the low fuel/reserve is directly related to the position of the float in the tank (the right one I think) and is nonadjustable.

As far as the different readings in the AM both tanks use a resistor on the floats to tell the gauge how much fuel is left. Lower battery voltages in the AM affect the resistors and will make the fuel gauges read low until the car gets up to full power, usually in about 3 to 5 miles.

As DavidS mentioned there is some smoothing going on to keep the needle from fluctuating too much and to calculate both tanks not just one as most cars have.


Joe
 

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Mine exhibited the same characteristics until . . .


One day I was out for some sporty driving in the rural roads of South Carolina when my engine died. No matter how far I pressed the accelerator, the car just continued to slow down until it eventually stopped. I thought I had torn up something with my amateurish driving skills. Gas gauge showed quarter of a tank.

After a flatbed trip to the dealer, they found a faulty sensor and pump. I HAD RUN OUT OF GAS!
 

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hudson said:
Jerry

I believe the yellow light for the low fuel/reserve is directly related to the position of the float in the tank (the right one I think) and is nonadjustable.
Ok, a very minor annoyance to be sure. I suspect there is some adjustment if I were to go into the tank and mess with the float, but I am not going there!

As far as the different readings in the AM both tanks use a resistor on the floats to tell the gauge how much fuel is left. Lower battery voltages in the AM affect the resistors and will make the fuel gauges read low until the car gets up to full power, usually in about 3 to 5 miles.
That is my experience, so it makes sense. Would hooking up a battery tender keep the gauge at the same place as when the car was shut off the night before? Maybe I will do a little experiment......

As DavidS mentioned there is some smoothing going on to keep the needle from fluctuating too much and to calculate both tanks not just one as most cars have.


Joe

That's a good thing, as the needle does not bounce around much at all.
Regards,
Jerry
 

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That is my experience, so it makes sense. Would hooking up a battery tender keep the gauge at the same place as when the car was shut off the night before? Maybe I will do a little experiment......
Jerry

You would have to leave the whole car powered up (key in no. 2 position) to get a true reading. All the contol units, wiring, resistors, and connectors generate a small amount of heat that would change the values a little but still noticable. But try it anyway just to see.

Joe
 

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Discussion Starter #15
hudson said:
Jerry

I believe the yellow light for the low fuel/reserve is directly related to the position of the float in the tank (the right one I think) and is nonadjustable.

As far as the different readings in the AM both tanks use a resistor on the floats to tell the gauge how much fuel is left. Lower battery voltages in the AM affect the resistors and will make the fuel gauges read low until the car gets up to full power, usually in about 3 to 5 miles.

As DavidS mentioned there is some smoothing going on to keep the needle from fluctuating too much and to calculate both tanks not just one as most cars have.


Joe
Joe,

Not a bad explination. I am 100% certain that every time I get into the car in the mornings, that the fuel guage reads lower for the 1st few miles. Now this is not such a bad thing, but if you go to the OBC reading for "miles left to empty" you will also see the same thing occur there as well. I can understand the OBC, but I wish it was different with the fuel guage.

Interesting info regarding the two fuel tanks. I did not know this.

Mark
 

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MAH said:
Joe,

Not a bad explination. I am 100% certain that every time I get into the car in the mornings, that the fuel guage reads lower for the 1st few miles. Now this is not such a bad thing, but if you go to the OBC reading for "miles left to empty" you will also see the same thing occur there as well. I can understand the OBC, but I wish it was different with the fuel guage.

Interesting info regarding the two fuel tanks. I did not know this.

Mark
Yeah Mark,

I've heard a few people comment on the same problem as well. But that's the nature of the beast. :cheers:

Joe
 

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Discussion Starter #17
hudson said:
Yeah Mark,

I've heard a few people comment on the same problem as well. But that's the nature of the beast. :cheers:

Joe
Joe,

Actually I was just kind of nitpicking. I still like the M5 as much as the 1st time I drove it off the lot.

Mark
 

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Timely post... This morning, as I started up the car, the gauge reserve light was ON and Range = 40 miles. My garage is level.

When I arrived at work 12 miles later (part city, part freeway), the light was OFF and Range = 55 miles (parking level again). This seems to support the "gauge reads lower when cold" theory, I guess. I'd never thought about it until today. Regardless, I decided I'd better fill up on the way home!

I have found that the "range" is very accurate on my car. On my drive across country, I got stuck in a big gap between towns in Colorado and had to nurse it to the next town. I made it there with Range = 12 miles!
 

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There should even be an margin, so 0 in remaining distanze and empty tank (gauge) will still allow you to search for a station nearby, i've tried, and it works :)

If it's good for the car or not is a different story.
 

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At WOT and full RPM the S62 will drink one liter of petrol in less than 26s.

The reserve below "zero fuel" seems to be about three liters. The light comes on with about 10 liters left.

Don´t you agree you should be able to drive more than a few minutes after the alert..? ;-)

David
 
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