Hello. First post on this site. I have a 92 240 sedan that has given me tons of problems. I have had to replace a rusted oil pan, flame trap, transmission cooling lines, put in a used MAF sensor, new idle control valve, new rotors, new calipers, used fuel pump relay, and just put in a new O2 sensor. Some of the problems that I have had have been stalling at speed, low idle, stalling at idle, and hesitation when I am driving from a stop sign or light. Thankfully the stalling problems seem to have stopped, the check engine light was on but stopped when I had the pan replaced. I still have low idle, not sure of rpms because I don't have a tachometer, and after I replaced the oxygen sensor I cleared the codes to see if the 2-3-1 code came back after driving. They came back, the hesitation is not as bad as it was before but it is still there, previously it felt like the car was going to stop in traffic but now it hesitates for a couple of seconds and then seems to catch. The interesting thing for me was that when I unplugged the o2 sensor it didn't have the hesitation any more, it did seem to lack power though. It is getting spark, the fuel relay is clicking when I turn the key, the pump sounds like it is working, I unplugged the fuel injectors to see if the engine was reacting, it was, now I am wondering what the next step should be? Could this be from a bad catalytic converter (the o2 sensor was real black when I took it out, although it might have been the original), should I try replacing the fuel filter, is there a way to adjust the rich/lean of the fuel mixture or the idle? Should I just unplug the o2 sensor and drive it? I have also replaced the fuel pressure regulator. Thanks for any help. Jay

 

I am having a simular problem with my 90 740 turbo.
I would bet that within this info below lies your answer.

There are many things that can make it run poorly and give a 231 code such as throttle switch,engine temperature sensor,air mass meter.I would check the signal for each sensor in the fuel system according to the repair manual and then check the wiring harness and connectors to each sensor for corossion.

 

Are the plugs black too?
The O2 sensor probably went bad due to rich running, the ECT sensor can do that.
Its strange that it isn't throwing you a code though.

My car hesitated and it turned out to be fuel pump, it ran fine at speed but starting off it bogged down.
I think the AMM sensor can cuase this too.

 

Problem seems to be fixed, no stalls or hesitation for two days now. Keeping my fingers crossed! Thanks for the suggestions. Seems to have been the o2 sensor all along.