Black plugs also suggest oil in the mixture. Oil can mean worn valve
guides/seals, and/or worn rings. I don't recall if you told us the mileage
on the car, but an '89 suggests high mileage. My 325 ('94 with the M50
motor) was closing in on 220,000 miles without any sign of being worn out.

I

Cap and Rotor AND wires. Don't forget the wires. I would be looking to these
components before the sensors because the sensors typically result in a
go-no-go scenario, not a goes-crappy-after-a-certain-point scenario.

I have substituted the airflow sensor from my other

A bad O2 Sensor can cause the motor to run rich. The coolant temp sensor can
also make the car run rich too. These two sensors adjust the fuel mixture,
and if they are telling the computer that the car is cold, then the computer
will set the fuel mixture as if the Choke (this is a part that carburetors
have, but fuel injection accomplishes by changing the injector timing) is
on. The choke is on when the engine is cold, and it causes a rich mixture
until the engine warms up.

I wouldn't place these two sensors at the top of the list for the trouble
you are complaining of, but they can certainly feed into the black condition
of your plugs. Assuming the plugs are fouled and you replace them and do
nothing else, the car should run reasonably well again until the conditions
caused by the O2 and Coolant Sensors to foul the plugs again. That is, if
the sensors are bad and they cause the plugs to foul, replacing the plugs
should immediately restore the engine again until the sensors foul the new
plugs.

If you had bad sensors AND trouble with the cap, rotor, and wires, then you
would still have a crappy running motor even if you replace the plugs and
sensors.

My Jeep recently began to run like sh.t, and two plugs had fouled. I have
converted my Jeep from a carb to fuel injection, but I still have a
distributor and plug wires. Anyway, I put in new plugs and the trouble
immediately went away, but looking at the plugs I can see that I'll be
needing valve guides and seals soon. My point is, you can have underlying
problems that won't have any day to day effect on the running of the engine,
but will manifest themselves over time. The time it takes decreases as the
problems go unattended. The point is, you can correct the affects of the
underlying problems and restore a semblance of normalcy. I think you
probably do need an O2 Sensor and a Coolant Temp Sensor, but these parts are
not part of the crappy running that you are chasing down.

Fuel Pressure Regulator fits the scenerio of instant rich mixture.  The fuel
pressure regulator really works as a pressure relief valve so when they fail
the pressure goes way up - maybe close to 100 psi depending on the condition
of your fuel pump.  Sometimes they can fail in a mode where it will
temporarily fix itself if the engine is turned off for a couple of minutes
to let the pressure bleed down so that might be another symtom.