This all started about a year ago when my 03 Cooper S got hit parked on the street. The kid did some real damage but his insurance ponied up and paid to get it fixed (~$13k all together). I sent it to the dealer for the repairs and one of the things they found was a crack fuel tank. They changed it in the course of the whole ordeal and it was returned after a few weeks.

The Problem:
When it came back it showed almost a full tank. My wife took it to work and ran out of gas. We got a couple gallons in it and took it to the gas station where it took almost 11 gallons to fill it up. Since it came back from the dealer, the lowest the fuel gauge ever got was about 3/4 tank. It read full just fine and she learned to fill up based on the odometer.

The First Attempt:
I opened up the fuel pump cover and started to investigate. Everything seemed fine, nothing was hanging up, and I put it back together. Nothing changed. Still never indicated below 3/4 tank. *****.

The Second Attempt:
Fast forward a year or so. For one reason or another I started driving the car and yesterday I found the inspiration to have another go. I did a little more research and found out there are 2 sensors for the fuel level. I metered the pins and one pair was around 100 Ohms, the other was closer to 500. I rang the wires up to the gauge cluster for continuity, it all looked good. No other gauge issues in the cluster so I assumed the voltage regulator inside was good. I said to hell with it and decided to rip everything apart. I pulled out the fuel pump and filter assembly from the other side.

The Solution:
Turns out the tech had installed the fuel filter assembly wrong and the float on the filter assembly was hanging up on the cross-over hose. While it was out it seemed like a good time to change the filter. It was pitch black after 112k miles. Quick trip to Mini got me the filter for $58. Came with all new o-rings and tank seals too. A little silicon grease and some time and I got everything back together. A little bit of a hiccup when it turns out I killed the battery, but easy enough. Now the fuel gauge is good and everything works like new. Horray!

The way BMW decided to make all this goofy **** work:
The fuel level sensors are supplied with about 10.5VDC from the cluster. It gets split off and pinned on different wires at the tank top connector (Brown wire with black tracer and white spots, pins 1 and 3). Each level sensor provides between 20 Ohms (empty) and 300 Ohms (full). Sensor 1 is on the left side and reads back on the black wire with red tracer and white spots, pin 4. Sensor 2 is on the right side and reads back on the black wire with red tracer and yellow spots, pin 6. Pins 2 and 5 drive the fuel pump. Inside the tank, there's a connector between sensor 1 and sensor 2 on the left side. Both tank level sensors should read about the same depending on tank level, within about 20 Ohms. The two sensor signals go back to the center gauge cluster left connector on pins 22 and 23. There they are averaged using creepy analog electronics. The average of the sensors drives the gauge. They are supplied voltage from pin 10 at the cluster. The fuel pump receives and intermitant 12VDC when commanded by the ECU (I guess).

Hope this helps anybody with weird fuel gauge issues or is at least a starting off point for trouble shooting some other problem. Special thanks to gknorr for his fuel pump change writeup -
https://www.northamericanmotoring.co...placement.html
and whoever posted the link for Mini wiring diagrams, super helpful at
http://pinoynapinoy.us/

Sorry everybody for the long post, but this has been driving me nuts and northamericanmotoring forums really helped me get to the bottom of it. Just wanted to report back in case anybody can use it.

 

I for one am surprised they didn't total an '03 R53 that needed $13k in repairs!

 

It ended up being a pretty bad hit. The kid came down my street and clipped the left rear corner. Initial estimate was about $8k, but the deeper they dug into it, the more they found destroyed. Suppose it's in for a penny, in for a pound. They ended up replacing the bumper cover, hatch, left side of the rear suspension, left rear wheel, left quarter panel, fuel tank, left exhaust and a whole lot of odds and ends. Worst part was she had to drive a Chevy Aveo for a month. Rental car guy actually said "They're both compacts, this is basically the same car." I think she wanted to choke him.

 

Yikes, an Aveo... that is rough!

Someone hit my car while it was parked, too... this was just a few months ago. $2,500 repair. Spraying the whole side of the car, pulling a dent out of the door, and replacing the mirror. Because BMW/MINI is a "premium brand," I got a 2012 Mercedes C250 as a loaner. I told them to take their time.

 

LOL

Life is ANALOG...all stuff have to start out analog...and CAN be converted to digital.
The beauty of analog is infinite number of Resistance level for a gauge, and you just need to display the Resistance on a meter..than label the max resistance and the min level full and empty!!
there are two sensors cause of the shape of the tank....hence, combining the signals to average them.
besides that I find it amazing the repair shop reused a 112k miles filter that was BLACK!!
You did some GREAT trouble shooting work!! Congrats!!

 

Oh, I get it. I'm an electrical engineer. I just hated analog electronics. And I see what they're trying to do. There's actually a small pump that shifts the fuel from one side to the other. I guess they were trying to make sure the fuel was uniformly distributed even when it's sloshing from side to side while cornering and make sure the fuel pump is always wet. Actually, the pickup is on the right and the pump is on the left. So it filters on one side, then transfers to the other side to build pressure, then goes back to the first side to send it out to the engine. As far as the filter, they just swapped the whole unit from the old tank to the new one. You have to disassemble the fuel filter unit to get to the filter.

 

My 2005 Mini fuel gage kind of works but reads high. At 1/2 a tank, it reads 7/8. I checked the car analyzer (gage test 6.1) and it concurs, saying it was down 2.5 gallons yet the car took 4.6 gallons to fill up.

Considering what was said above, it seems that there is additional resistance somewhere.

Does anybody know where connector X15 is? Somewhere between the instrument cluster and the fuel level sender.

Thanks,
Peter