Sorry I forgot who posted, but someone with a 90 model e34 has had mpg and range problems with the obc. Check the gas gauge, does it show readings between full and 1/2 or go to half very quickly? Cars produced 6/90 to 2/91 had a non linear gas gauge that must be corrected by a software download from the dealer. US service bulletin 62 02 91 (3237). If you think this might be the problem, let me know and i will share more details.