I have a 1991 740 Turbo that is missing at idle. # 4 is not firing. I have checked the plug, plug wire, replaced the injector and still no response. It seems to clear up above 1000 rpm. fuel economy is off but i really don't notice a power difference.
either unplugging the plug wire or the injector wire has no effect on the engine at idle. It's just a dead miss.

 

Check your compression on that cylinder and the one next to it.