code 44

My sister inlaws 89 S10 is driven me nuts, I keep pulling up code 44 on it. this is a lean exhaust indicated ,according to the code book.but her gas mileage droped from 15 to about 9 miles per gallon.replaced throttle body,throttle position sensor,rebuilt fuel pressure regulator,and replaced fuel filter.installed new wires and plugs about 5,ooo miles ago oh yea new map sensor too.How can it be using more fuel but still come up with a lean exhaust.Did not find any vacum leaks .Could this be a bad 02 sensor  feeding back the wrong info.Some days it will not start,smells flooded. When it does start first thing in the morning it blows out black smoke and runs rough or shuts off if you do not keep on the gas.Thanks for any input (Steven Daub)

Certainly could be a wonky O2 sensor or an exhaust leak before the sensor.

It was the o2 sensor, no more service engine soon light. yeaaaaaaa.Now if I can figure out the hesitation rightt off idle when you first pull out ,i'll be set.