Something must be causing your plugs to foul, though.

A wet or dry plug fouling?

Does your RWD 90 series have Turbo? How many miles?

Something engine control has not set as a fault code yet your engine is running rich.

You have verified proper ground and power connection between the battery and alternator to the engine. Poor power connection can tend to a rich running condition.

Fuel rail pressure regulator? Though you would get rich or lean / rich running fault codes from Motronic.

You have tested sensors like both 02 sensors or the ECT sensors for function? Or the AMM / MAF? Though, again, Motronic would set codes, I guess. I have no idea the fault tolerance or 1998 S and V RWD 90 engine control fault tolerance.

On 1989+ 240 is can be rather forgiving, yet not so far as plug fouling. Tho the single ignition power stage (you have two in S & V RWD 90 series) is not monitored for OBD fault, yet connector corrosion can halt the engine. Maybe before hand one would have weak spark to the coil (or coils in your instance).

Just a guess fest. Celebratory in spirit, certainly!

Hope that halps.
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