Fuel gauge does not work in my 1990 325i BMW

run the fault codes and look for a venting problem in the EVAP system

Fuel gauges are not meant to be precision instruments. You might want to fill up a 2 gallon can and drive the car until it runs out of fuel to see how low the gauge will go before it's really empty. Use the 2 gallons to get back to a filling station.
I once spent three working days trying to get a 1972 Chrysler to read full when it's full, half when it's half and quarter tank when the tank has a quarter amount of fuel. The customer did not want to accept anything else, but in the end all I did was to get close, not exact.
It is possible that the fuel float in the gas tank is "drowned", that is, the plastic float might have a slow leak and some fuel found its way inside. This will cause the float to submerge slightly and give you a low reading. To check this you will have to remove the float unit from the tank.
In any case, if you feel up the job (not easy) remove the float unit from the tank and manually move it up and down while checking the meter reading. You can bend the wire holding the float to correct misreadings.
Personally, I wouldn't bother, just be aware that when the reading is low, feel safe that you still have enough fuel to get to the next filling station.

replace front fuel pump because gauge is made into fuel pump and not serviceable. Or poor boy way of getting by would be fill front tank with gas reset mileage drive normal for a day. refill again with gas note gallons used and mileage driven for average miles per gallon. front tank should be around 17 gallons multiply by mile per gallon you figured

mine did the same thing 2 years ago. i assume it is probably the fuel gauge sending unit in the fuel tank so i priced a new one. as i remember it was about 350.00 for just the part. my computer reads in gallons and is still accurate as to telling me how much fuel i have used so when i use 10 to 12 gallons out of the tank i fill it again. so far i have had no problem doing it like that. just don't run out of fuel because that is really bad for fuel pumps. mine has a 14 gallon tank so i make sure that i never go below 12 gallons used. i usually try to fill at 10 gallons used

Your gas tank should hold about 13.1 gal.s I believe and your oil for a oil change including filter is 4 quarts only....don't overfill! I own one so I know.

There is nothing else to replace ... presuming you did everything correctly and the replacement parts were not faulty, I suggest you live with it. Keep track of the miles you drive and fill up when you have driven about 1/2 the estimated miles per tank for best results. I.E.: you get 10 MPG - your tank is 20 gallons = 200 miles per tank ... fill up every 100 or so miles.

OR

You could take it to the BMW place and let them fix it.

Thanks for your question @ FixYa.com

hi it can really only be two things, first is the fuel sender on the pump inside the tank it could be stuffed or sticking causing a false reading, second it can also be a faulty fuel gauge on the dash.

First, just because ther is fuel present, doesn't mean that there is enough fuel pressure present to fire your injectors. The fuel pressure specification on an M54 Engine is Bar 3.5 at a delivery rate of 2.25 Liters Per Minute. (or in U.S. terms, 51 PSI at 0.6 Gallons Per Minute) This must be measured with a gauge to make sure it is correct before determining if there is enough fuel, or is the fuel pump is working properly.

Second, did you check for spark at the spark plugs?

Find out how many gallons your fuel tank holds. Once it falls below a 1/4 tank and starts to shutter. Proceed to the nearest gas station and fill up. If you fill it up with 12 gallons and your tank holds 12 gallons then your fuel gauge is off. So it should be reading empty instead a 1/4 tank.

Those vehicles have to sending units. Your fuel signal is buferred ( calculate) so it takes both signals to make your gauge accurate. sounds like one them is out of range causing your gauge to read icorrect. You do need to take to your nearest BMW shop, so it can be properly diagnose.