1987 528e poor gas mileage

Have you actually checked the mileage by refilling the tank and using the gallons shown on the pump? If not, my first guess would be a bad fuel gauge.

Frank

Yea I did check it by filling. The car is a damn pig.

malfunctioning 02 sensor?

It could be stuck in cold start mode due to a bad temp sensor (or wiring) and running rich.

Morronic equiped cars do not like badly worn distributor caps / rotor arms. Replacing them could result in much better mpg and high end performance.

It's also not impossible you have a stuck fuel pressure regulator. Stick a glass bottle on it's output and see if you get any fuel coming through.

You ought to be getting at LEAST high 20 or low 30s mpg.

My stepdad was on vacation, hes a mechanic. He'll take a look at it tomorrow. I dont know anything about cars.