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1990 Deville 4.5 L decreasing fuel economy

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I seem to be having trouble finding this exact topic, so I'll try here....I have a 1990 Sedan DeVille with the 4.5L V-8, have owned for about a year. Bought it with 47k original miles, it had been well maintained and garaged. So my fuel economy has been as expected about 18 city, maybe 25 highway. Idles nice and smooth. PLENTY of power! It now has 51k miles. Over the past few months, the exhaust smells rich, and my mileage has decreased about 20-25%. Another symptom is, it seems it just doesn't want to shift into overdrive, which would explain bad mileage, and then at idle, it has the rich smelling exhaust. The car idles nice and smooth, so I think leaking injectors are not an issue (I think they may have been replaced, somewhere in the past). I just wondered if you guys had any thoughts, maybe the Oxygen sensor is defective? There are no warning lights, and no codes "thrown", but if I monitor the OBD it does seem to indicate a rich fuel mixture, and I can observe the OD not engaging properly, at times. Any thoughts? Thanks in advance for your help!

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