Fuel Gauge Not Functioning Properly

[RossoDiamante]

Just got my new (to me) m400 and have discovered a host of things that need to be addressed. One of the more urgent things is the fuel gauge. When the car is off, the fuel gauge reads 0 (empty). However, when the car is turned on, the fuel gauge immediately jumps to 1+ (extra full). When I took delivery of the car off the transport truck, I noticed to full gas reading and thought to myself - my, how considerate that they filled the tank prior to shipping it. So I drove it around for a day but noticed that the fuel gauge never moved. So I'm thinking that this car either as a 490 gallon tank and that it just hasn't used much fuel or the gauge is broken. Being more suspicious of the latter, and worried that I might be running on fumes, I went to ye olde friendly fuel refilling depot and lo and behold she sucked up 13+ gallons of fuel! BTW, what is the capacity of the fuel tank; how close to fumes was I?

If I had to guess, there are just 3 components to this system. 1) fuel level sensor 2) wiring between sensor and gauge and 3) gauge. Is there anything else? If I had to guess, I'd guess its the sensor that is bad. Has anyone else run across this? What is the troubleshooting algorithm I should follow to diagnosis and correct this?

Thanks,
David

[andygtt]

its possible the wiring to the sensor in the tank has come off... its very hard to get to and is at the top of the tank near the bulkhead centre side of the tank (diagonal to the fuel filler point).

My fuel gauge did this on 2 different times and it turned out to be a break in the wire the first time and the actual sensor in the tank getting caught and not moving the second time. I changed the tyre of sensor in the end to a better type.

if your sensor is at fault you have to remove the fuel tank to replace

[RossoDiamante]

I'll hope it is the wiring then. I'll be able to take a closer look at it tomorrow. Jiggling my wheel speed sensor wire fixed my speedometer/odometer. Perhaps I will be just as lucky with this!

[andygtt]

RossoDiamante said:

I'll hope it is the wiring then. I'll be able to take a closer look at it tomorrow. Jiggling my wheel speed sensor wire fixed my speedometer/odometer. Perhaps I will be just as lucky with this!

Oh, worth mentioning that there is a plug that goes to the sensor in the engine bay that can come undone (did on mine and it read full like yours)... its near the front turbo so have fun locating it.

Thing is, the fuel level is something you want to work.. or not work.... you don't want it 'temperamental' as you end up running out, as I did a few time

[RossoDiamante]

Absolutely! I'll report back if I find anything interesting.

[Driven]

If you do have to remove the fuel tank, remember it comes out from the bottom of the car. You remove the under tray / belly pan and go from there.

[RossoDiamante]

Makes me long for a lift!

Catch-22: I assume you'd want to fuel tank empty (or nearly so) before dropping it (weight plus needing to work with it). But with a non-functioning gauge, you have no idea how much fuel is left in there. So you drive it and hopefully don't run out of gas!

It appears there are two wires that are on top of the tank that are attached to two spade plugs there. The one attached to the plug towards the front of the car is brown and the one attached to the plug towards the rear of the car is white/green. Is there any sort of polarity associated with these wires? Could they just be reversed? Or is it just a resistance output of some sort? Does anyone know the relevant reading range of this sensor?

I did find the connector for that wire. It was connected. And the wiring seemed to be intact from the sensor, through the connector, up to a point where it seems to go under/behind the fuel tank again. Wasn't able to trace it farther back.

[Jim O.]

It's easy enough to drain the fuel tank - no need to drive it around until it runs out of gas. You can also check the gas level by dipping a stick down through the filler hole. If you really want, you can use the volume and height of the tank to calibrate your stick, and figure out exactly how much fuel is in there.

This is pretty basic, but it still needs to be pointed out - to drop the tank out the bottom of the car, you will need to raise the car up at least the height of the tank. You'll need a pretty tall set of jack stands to safely support the car that high. A lot of floor jacks won't go that high, either...

[kenloen]

yes the sender unit is a variable resistor on a float device. you can remove the sender to check or replace by dropping the fuel tank about 6", no need to fully take the tank out.