Gas mileage isn't bad, but I haven't done a ton of driving so it's hard to say, but likely around 18mpg or slightly higher.

Vacuum lines have been replaced with new hoses and charge pipes/turbo hoses are all tight.

Checked out live data from the O2 sensor via OBDII and got the attached results after the engine was warm. There was only one sensor available for O2 sensor, but I have heard the upstream is the only one you can see data on, the downstream is just feedback. The O2 sensor seems to be working well per the voltage graph

, but the comment on "fault in the feedback system" has me wondering if the downstream O2 could be causing the issues?

 

Exactly what "issue" do you have?

You have a 40 year old car with I guess the original engine that still runs and you get 18 (normal) mpg.

Certainly your front O2 sensor reading are normal. Your "visible exhaust smoke" could easily be (here again a 40 year old engine) from worn out stuff in the engine. Just be happy you have wheels!

To solve your "visible exhaust smoke" might require more $$$ than the car is worth. Sorry.

 

could be a leaky injector Try running some injector cleaner like Techron or a brand name 93 octane for the next few tankfuls. Or you can have a service test/clean the injectors. PS I do have a set of used Bosch orange injectors you can try swapping in (bought them as spares for my 850T but sold the car.) PM me if interested Note I have some other 850 parts that I was gonna toss as well if you are interested, ( not asking for $$ just a place to ship)