Fuel Gauge Errors

[wingding44]

My ‘97 1.9L Z3’s fuel gauge will indicate erroneously, let me know if you have any ideas:
- Gauge needle will show beyond full for approx the first 125 mi
- Will then slowly reduce over the next 100-150 or so miles to half-tank
- From indicating half-tank to indicating empty it can’t be more than maybe 50 miles or so. You can almost watch the needle move once it hits the half-tank mark.
Any ideas on where to start looking for a culprit? Thanks for the help!!

 

[Nodzed]

That's normal for a Z3

 

[Zephyr]

Had seen this on my E36 back in the day.
had to swap the whole instrument cluster, never found what was wrong with it.
and then I had another issue as the odometer showed 120.000 km more than what the car had. (264.000 total).
had to reprogram the car and it was not cheap.
If you decide to replace the whole panel, try to find one that has similar or close total mileage.

 

[Mike Fishwick]

Yes - the more the needle goes down, the faster it goes down! I work on the basis that with a 12 gallon tank and an average thirst of 30 mpg, the shortest distance between refills on a run will be about 350 miles - but I never push it! I generally refill when the guage shows about a quarter full, which leaves plenty of reserve capacity.

BMW Service instructions - which tell dealers what is wrong with their cars - reccommend fitting a fuel level sensor with a gold film track, but only in cases of 'repeated customer complaint' . . . in other words, try to fob off the customer for as long as possible!

I am in the habit of always filling to the top, and calculating the average consumption, which will give you a good idea of your car's consumption in a given type of driving. I have also calibrated the MFA fuel readout, so it is within plus or minus 2 mpg, try to drive economically on a run, and find the fuel which gives the best mpg.

On my last few UK trips (a few years ago now) it was always Esso Supreme 97 octane. Here in France I use the local supermarket's 98 octane, which gives about 35 mpg at an 80 mph cruise. The best French fuel is total Excellium 98, which can give up to 40 mpg. The worst mpg has always been Shell V-Power, which averages 28 mpg at 80 mph in France, or 25 mpg even in the UK motorway procession.

 

[Pingu]

I have fitted a different fuel gauge, so I have a similar problem. I intend to...

Empty the fuel tank
Remove the fuel level sensor
Move the sensor to see how it affects the gauge
Clean the sensor
Try again
Use appropriate resistors to get the gauge to read correctly. (Full = FULL and Empty = EMPTY) I won't be able to do more than that, as the sensor's resistance profile should match the profile of the tank. If it doesn't, the gauge will never match the actual level or volume.

The level and the volume profiles will be different, but looking at the Z3 tank, the profiles won't be VERY different.

Examples to show how the shape of the tank's sides can affect the reading if a linear variable resistor is used...
If the tank is a cylinder on it's end, the two profiles would be the same. 90% volume = 90% full, 80% volume = 80% full, 50% volume = 50% full, etc.
If the tank is a cone with the point at the top, 90% volume = 53.6% full, 80% volume = 41.5% full, 50% volume = 20.6% full, 10% volume = 3.5% full
If the tank is a cone with the point at the bottom, 90% volume = 96.6% full, 80% volume = 92.8% full, 50% volume = 79.4% full, 10% volume = 46.4% full

 

[137699]

Totally normal - mine is exactly the same

My average long distance journey...

Full... full... full... full... nearly full.... empty... REFUEL NOW!!!

 

[ExMX5owner]

I had an issue with mine, very similar, It appeared that the unit had been removed prior to my ownership, and the whole unit had been replaced scewed. putting it back to how it should be did improve the situation. but it still falls rapidly past about half a tank on the guage..