did you install that sensor correctly? I friend of mine had that same reading scenario until he replaced his O2 sensor.

Regarding injectors .. I replaced mine and it made all the difference. I had idle issues and rough running .. did everything else one would try and finally replacing the injectors sorted me out proper. With that much mileage you may as well go for it if you're planning on keeping the car. It's an easy job and not too expensive. Once you have new injectors in you can safely eliminate them as a variable. Cold starts could be cold start valve, try to get one at the JY.

Test your vacuum with a gauge .. don't bother looking for leaks, just check what vacuum you're pulling at the intake manifold. Anything over 15/16 is pretty good IIRC. I would look for running problems elsewhere if you have sufficient vacuum at the manifold.

Replace your fuel filter too.
Looks like the car has had a lot of maintenance, that's a good thing. You're lucky you have those records. Car sounds like a keeper.

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1992 190e 2.6