MPG has dropped from about 21 MPG to 18 MPG.
The check-engine light went on following the below work.
Performance improved after the work, but MPG dropped to 16 MPG.
After a week or so, I reset ( ? ) the check engine light by removing and re-inserting fuses 1 –11.
After resetting the check engine light, MPG went to 18 MPG.
Proper voltage output from the Oxygen sensor was verified.

What is normal MPG?
What are the likely causes of the reduced MPG?

Recent work:
About a month ago, I replaced the harmonic balancer because it was slipping and making noise. I marked the inner and outer portions of the unit with paint to confirm it was slipping and making a lot of squeak noises.

I restarted the engine with none of belts attached and noticed a vacuum noise. I determined the intake manifold gasket to # 2 cylinder was leaking. Apparently, the intake manifold nuts to #1 & 2 had been over tightened by the previous owner. This pressure over time caused the gasket to become plastic and deform from an “O” to an eclipse. I replaced the gasket without disconnecting linkage and hoses to the intake manifold and throttle body. I used a lot of carburetor cleaner to clean the head and manifold. It was about 20 degrees Fahrenheit in the garage and it is certain there was an accumulation of carburetor cleaner in the head and manifold. The engine acted as if it was flooded and upon starting the check-engine light was on.

Our 1990 740 turbo has about 135,000 on the original engine.

Thanks

Curt