Odd gas issue on 1991 318is

[e3091318is]

I just finished my rebuild not too long ago and took the car out for the first long distance drive (75 miles-ish or so) a couple days ago. The fuel gauge showed nearly a full tank (the line before full) and after the appropriate amount of driving the fuel gauge read 1/2 tank. I was on the freeway and the car just gave out and the gas pedal wasn't doing anything. I coasted off the freeway into a parking lot and the car wouldn't start properly, just sputter and die out. I thought maybe the fuel pump, but it turns out I was out of gas! I filled the tank and she started up again with no problem. Now here is were it gets odd. When I filled the tank it stopped at 11.5 gallons. I continued to try and fill more thinking it was a 14.5 gallon tank and it started overflowing at 12.2 gallons. The gauge read a little past a full tank (needle past the dot by about a 1/16th of an inch). I have put 100 miles in city/highway mixed, and the gauge now reads about 3/4. Im not sure if my gauge is not working however I replaced it with another used, or perhaps something was stuck from the car sitting around so long without being driven. No orange light came on when I ran out of fuel either.

[monty23psk]

my 91 318is has a 14.5 gallon tank. At least that is what it states in the owners manual. Other e30s had the larger 16 gallon tank. Everytime I fill up, which is when the light turns on, I can never get to 13, always 12.1 to 12.5. The light turns on when there is about 1.5 gallons left. In your case, either you gauge is out of calibration or most likely the fuel sender unit in the tank is bad. If you said car was sitting a while, I would vote on your sending unit.

[e3091318is]

Yea that is what I am thinking.  The gauge tends to go down to where it is supposed to and then going back up to full.

[DesktopDave]

Could be like my car - I have a few big dents in the bottom of the tank.  Mine maxes out at about 11 gallons.  The sender makes the gauge pretty pokey too - it is often under by as much as 1/8 tank, occasionally as much as 1/4.  When my reserve light blinks on I have space for about nine gallons.   

Those senders can't last forever - it's a variable resistor.  More likely, if the tank or breather pipe was open to the atmosphere (while you were doing the rebuild) keep in mind the tank will condense water fairly well, especially if your fuel has high amounts of alcohol.  I've had all kinds of fun with motorcycle tanks and high-alcohol fuel...