My 1990 240 has been running like crap. Rough idle, intermittent stalling and hesitation at various speeds, black soot from tailpipe and on plugs. I've owned it for 7 years and never had any real issues I couldn't solve on my own. The check engine light came on two months ago, and was showing codes for all sorts of things, including the O2 sensor and coolant temp sensor. After three trips to a noteworthy shop (and $1000 later. Woof.), no improvement. Items I replaced/cleaned include:
Both fuel pumps
Coolant temp sensor
O2 sensor
Throttle body refurb
Full tune up
Cleaned air mass sensor...
Though I'm a bit embarrassed to admit it, after dumping all that cash, what finally proved to be the culprit was that 25 Amp fuse next to the coil. At the time I had no idea what that fuse was. It's my understanding that it's directly involved with the fuel injection system. (Someone please correct me if I'm wrong!) I had looked at the fuse several times and noted that it wasn't blown. Then I pulled it out and it was filthy, leads corroded. I cleaned the contacts, replaced the fuse and BANG! Engine light turned off, idle was smooth and balanced, runs great. I joined this forum just to share what I learned (the hard way). I'd recommend starting here if you're having the same issues.

 

Thanks for your message! I am a relatively new owner of 92 240. It's helpful to get insider information here.