Rich running '88 911

I need advice. My 88 runs extremely rich. You can see the smoke and smell it when it idles. I have read the 2 common sources of info on this fuel injection system: 'Bentley' and 'Bosch Fuel Injection & Engine Management'. In the Bosch book it says the output from the oxygen sensor should be around 500 mv and that will correspond to optimum fuel/air mixture. In the Bentley it says the voltage will be 'fluctuating' between 100 and 900 mv. Which is right? I don't have a CO meter so right now I am trying to determine if I have a bad oxygen sensor or control unit. Any suggestions? Thanks.

The O2 sensor doesn't typically make that much of a difference.
Test the cyl head temp sensor resistance hot and cold (bentley should have the specs) with an Ohmmeter.
CHT sensor failure on the Carrera is pretty common, and can cause the car to run so rich that it can't even start.