My 1990 740GL is getting v8-like mileage in the teens. Yes, pretty sad, but what i'm wondering is whether or not my new oxygen sensor is working properly. Before I replaced the O2 sensor, it read a constant .7 to .8. So i thought it was broken and i ended up putting a new one in. Now it reads the exact same .7 to .8.

Could it be that the o2 sensor is not broken, but merely reading a rich running engine that cant be leaned out (and therefore doesn't cause the o2 sensor voltage to fluctuate down to, say, .2)?

Can a vacuum leak cause permanently rich running scenarios? Or possibly even leaky injector seals? I've wasted a lot of $$ in gas, and I'm ready to bring the mileage back into the mid 20s.