I have a similar problem in my '91 735i except in my case at fillup the gauge registers at full but then it rapidly moves toward 1/4 tank and stays there. In other words, after about 50 miles or so after fillup the gauge shows 1/4 tank even though the tank is probably 3/4 or more full. I had the sender checked and it appears normal. Should it be replaced with a new one? If so, about how much does a sender unit cost?