I've got an 06 M35x with some weird problem with the fuel gauge. A couple months ago, it was almost down to nothing on the commute home but then slowly crept back up to 1/4 tank over a couple minutes. I ended up running out of gas later that day. I had noticed something funny with the fuel levels in the weeks prior like I thought there was more or less in the tank the day previously but hadn't been able to confirm it until I saw the need going up while I was driving. Now, my gauge pretty much only goes between full and 1/2 tank (I think, I only drive about 200 miles and refuel).

Last weekend I finally go around to taking a look at it. I tried the method where you clean the contacts on the fuel level sensor, they were a bit dirty but it didn't change anything. The tanks should have had about 5 gallons in there. The driver's side tank with just the sensor was really low but the passenger's side had a lot more. Is this normal? I figured the tanks were connected to even out or get drawn from at the same rate but maybe the pump pulls from one then the other, I don't know. I don't think I would be able to get to 200 miles on 9 gallons in this car so I must be drawing from both (I average like 16mpg). I measured the sensors with an ohmmeter and got what sounded like reasonable numbers but I don't know for sure what they should be. The way the fuel gauge reads it looks like one tank is reading full and the other one is reading the normal level.

I also read about a possible instrument cluster problem since the sensor signals feed back to it. I tried the self diagnostic test from the service manual but that didn't really do anything. The needle works, it starts off at E and goes up to somewhere between half and full when you turn the car on.

Looking at the schematic, I see that the fuel sensors are in series. If one of the sensors had a short or was stuck, it seems to me like that would cause this weird "half tank plus half of what you actually have in the tank" scenario.

Can't find much of this for the 2006 M35. The 03/04 M45 seemed to have instrument panel issues and the 06 G35/37 seems to have fuel sensor issues but can't finding anyone with an M with the same problem.

I do recall the Y34 issues but you might be the first one I've heard of in the years here to have this happen on a Y50. Unfortunately.

I was thinking about it more last night and I feel like this could be a problem with either the harness or the connectors for the sensors. I just now noticed the service manual does have some numbers to check for the resistance of the sensors and some of the harness. If the gauge is always acting like one tank is full, then it could mean that one of the sensors has some sort of short or is otherwise stuck at some value. I did not check out the resistance at the connector, I only checked it at the actual solder joint for the wires. The numbers I got after cleaning the pads seem like they are in line although I can't recall the exact numbers and in this case, the exact number is what matters.

I've got too much other stuff on this car to fix before my next inspection: new u-joint on prop shaft, patch hole in exhaust, new front tires and then all of the other things: new A/C blower relay, valve cover gaskets, and now this fuel tank madness.