Gas needle acting weird

[Breeze1]

Here we go again. To each week its issue, at least my weekends are not boring lol.

So my gas gauge started acting weird last weekend, showing either what I know to be the accurate level of fuel, or about a quarter of a tank higher. Today I pulled up and verified both gauges under the rear seat and they seemed fine since when I moved each one of them respectively to full/empty tank positions, the needle on the dashboard moved accordingly.

Put everything back in place, needle back where it should be, test drove the car a few miles, turned it off and on several times, all was well.

I thought maybe the gauge had gotten stuck at some point for some reason and was happy enough for the "easy fix".

Now after taking the car for a drive it started acting crazy again. About a quarter tank more that should be indicated, and the needle slowly goes back where it should be. Same pattern as before: higher than normal, slowly and steadily back in place. No "jerkiness", always higher than normal, never lower.

Any ideas?

[EconomicHustle]

Bump because my E21 does the same. Just shows accurate or about an eighth or quarter lower.

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[bmwdirtracer]

Fuel level senders are based on resistance readings. The arm sweeps across a metal grid with different resistance at each end. If the resistance of the grid, or the wire terminal, or the wire itself, or the plug into the back of the instrument cluster, changes resistance, then the wrong information is relayed to the gauge.

The fault can be anywhere from the in-tank unit, to inside the cluster itself.

On the E46, there is a cluster self-test which will show the amounts of fuel the cluster "sees" in each half of the tank. While this is a nice piece of data, it probably won't help you discern exactly where the data is erring.

On the E21, I'd probably recommend replacing the sensor unit in the tank. While you're there, see if you see rust and crud in the metal tank.

[Breeze1]

Thanks Chris. I have tried unplugging the battery's negative lead for about an hour hoping that it would help getting rid of some random electric gismo. After plugging it back things are back to normal. I'll see how it behaves tomorrow.
I do remember smelling some electric burnt smell twice recently. I had dismissed it thinking it was coming from outside or something, but now that I've had this issue and if it comes back I'll have look at the cluster (and perform the self test no matter what).

[bmwdirtracer]

On E46 and later cars, disconnecting the battery does nothing more than resetting the clock.

(Disclaimer: Okay, on MOST system vehicles, it makes the MOST system reboot and look for new modules...in case the TCU was replaced, for instance. This has nothing to do with E46's or with anything more important than the telephone.)

For a burned smell, I'd personally look for oil on the exhaust, and a leaking valve cover. There's no way the fuel level measurement is causing a smell.

[Breeze1]

It was not an oil burnt smell, more like electrical wires/short circuit smell. Anyway I hope I get lucky on this one and that the gauge keeps behaving normally.
I don't feel like chasing this thing around to find the cause and I'd hate to not know how much fuel really is in the tank.

[MarcoZandrini]

Poke you head and your nose under the dash. Also, check the ground connection at the dash. As I remember it’s a knurled knob to the right of the steering column. The temperature gauge on my E21 was erratic (all others were fine). I flushed the cooling system, removed the radiator and cleaned it, all to no avail. Found the slightly loose ground knob. Fixed.

[Breeze1]

Poke you head and your nose under the dash. Also, check the ground connection at the dash. As I remember it’s a knurled knob to the right of the steering column. The temperature gauge on my E21 was erratic (all others were fine). I flushed the cooling system, removed the radiator and cleaned it, all to no avail. Found the slightly loose ground knob. Fixed.

I'll try that, since the issue has unfortunately come back. I also noticed that on 3 occasions, the reading was fine right after unplugging the negative lead of the battery, but would get higher than normal a few miles later. I'm not familiar with electrical systems at all, but that makes me think disconnecting the battery could temporarily fix the issue by draining some "faulty" current or something.

Thanks!