Sorry for the long post in advance, but I want to make sure I have all the symptoms covered. I have a 1987 535i with about 184k on the clock. Car was running great and getting around 20 mpg around town up until about 6 months ago when it refused to start. Diagnosed that problem down to an aftermarket alarm a from a previous owner that was preventing the fuel pump relay from turning on. Unplugged the alarm box and car started up but started running very rich. The car sat for about 6 months and I've just recently begun using it as my daily driver again, and the mileage has now dropped to 7-8mpg with around 80% non-aggressive freeway driving.

Ok, i know that was long, but here are the more recent details:

O2 sensor replaced 4 days ago, bought from BMP design. New one has a longer cable by about 1.5 feet, but socket matches up.

OBC and dash economy meter read 20-22 mpg on average, yet the car is getting around 7-8mpg.

Averaging about 90-100 miles before the gas gauge hits the 1/4 mark from a full tank.

Car runs terrifically, has plenty of power, and were it not for the fuel economy I wouldn't think a thing was wrong with it.

Car has (as far as I know) stock injectors, with no performance modifications at all.

Any thoughts on what else I could replace and/or adjust to get me back up to a reasonable amount of fuel consumption? Any ideas would be appreciated, this car is costing me about $100 dollars a week in gas alone, and that's just driving roughly 25 miles every day. Thanks in advance all.

-Nate in Phoenix
1987 535i