1991 740 Turbo
Engine is rough at idle, beginning a few weeks ago.
I have determined that #4 cylinder is the problem. If I disconnect the plug wire, no difference. All other cylinders, big difference. I exchanged plugs between 3 and 4 and problem continued. Changed plug wires and problem continued. Plug tip (which is new) showed slight carbon but looked like the others. There is spark from the distrubutor. It is not noticable at higher rpm, and performs normal when driving.
Any ideas would be appreciated.
Thanks

 

Possibly either a bad plug wire or a bad injector.

 

to test the injector... use a noid light that you can buy from ipdusa.com to see if the light flashes which means the injector is working.

 

The noid light will only tell you that you have power to it.If you unplug the plug with it running and the engine doesn't drop any in idle then the plug isn't getting spark or the injector isn't injecting fuel.