Yeah we see a lot of that, When I had the shop set up at my old place it wasnt unusual to get 2 or 3 weekend drop in visitors armed with mechanic horror stories seeking help or advice.
Air control valves being the usual culprit followed by setting a base tune to undo the fiddling people did to bandaid the fault instead of fixing it.
Air/Fuel on D jet is controlled by this,(Manifold pressure sensor) it can be refurbed and adjusted DIY or professionally done by Bosch for $400 to 500.
This is a K jet AAV (Aux air valve) and its thermo couple activated by oil in the brass bulb which is immersed in the intake manifold water jacket. (Same for D jet but with different angles on the hose)
These suckers usually gunk up and stick open, can be cleaned out with soak and air blast over a few hours and the bulb at the bottom squished/crimped a bit if they close too late at temps above 62.
WUR or Warm up reg which controls K jet cold start jet metering and fuel pressure, usually not a refurb item unless you are keen but they can also unlock performance on K jet cars through higher fuel pressure etc (Google DKUBUS)
