1991 750IL - Check Engine & Transmission Program
 

I have 750IL of 1991 w/ 109K miles. Under 40 MPH, "Engine Check" warning
light stays on. At the first starting of engine, throtle valve doesn't open
fully and slugglish slow. Then, "Transmission Program" message comes on.
Then I have to trun off the ignition key and wait 1 min.and turn again.
Then I can speed up in full speed. Engin runs rich gasoline smell and I
have to open windows to run. Computer test says C9(?) --> O2 sensor
problem. So, I replaced 12 spark plugs and 2 Oxygen sensors.But still the
"Check Engine" and "Transmission ..." come on. BMW dealer tech guy says,
engine computer or the comminication box between the engine computers to
exchange data from the each engine( 2 engines of 6 cyl= 12 cyl on this car)
for coordinating the 12 firing sequence might not be working even though the
transmission might be OK. So, if the engine problem is fixed, the
transmission message might go away also.
Does anybody know this issue and could you tell what to do and how to fix?
Thank you a lot in ahead.