850 troubles

You´re likely on one bank only. Pull AFM plugs to see which bank is not running. If only on one bank then pulling the AFM plug for the bank running will normally stall the engine whereas the AFM plug for the bank not running won´t change anything.
Did the Magnecore come with new donuts for #6 and #12 cyls? Hooked them up correctly? (easy to get confused there!)
Do you have the EML light on the dash lit? If yes, check/recondition your throttle bodies.
You´ll get there
Cheers
Reinhard

The "donuts" are the donut shaped cylinder detection sensors which check whether there is spark on #´s 6 and 12. You find them on the very beginning of the leads going to cyls 6 and 12. If there is a condition (ie missing spark, broken connector, malfunctioning sensor) then the DME monitoring this sensor in turn cuts the fuel for the whole bank and brings you into limp mode. You´re down to 6 cyls running and despite initial marketing blurb you cannot rush to the next workshop with 125mph but rather hope for not being run over by some agricultural appliance.
The connectors for the cylinder detection sensors and the impulse sensors are located on the front of the engine just underneath the oil filler cap. It is quite easy to mix them up since to my knowledge all 4 share the same connector type.

See attached for orientation and troubleshooting.

Cheers
Reinhard

wayneblackwell1000: That depends... The E31 DME and EML are not really known for their good diagnostic abilities. For example, accidentally swapping the cylinder identification and crankshaft sensors makes the bank fail to run, but no errors are stored. Other malfunctions may store error messages that at first sight hint at something completely unrelated. However in general I would say, yes, the fault codes are useful.

But... BMW INPA (which you have) doesn't support the pre-EWS M70 like in your 850i. Member quest at bimmerforums.com hacked the INPA M70 DME files, but so far the results have been mixed. It's worth a try (as in it won't damage anything), but it may not work for you. Normally you would use BMW DIS for this, but DIS is not native Windows software and either requires either a dedicated laptop or virtual machine...

Alternatively you can try to fetch error codes through the "stomp test". For this to work on a European E31, you will need to pull the instrument cluster and install the check engine light bulb.