Bad O2 sensor will give you a fixed 50% (open loop mode) signal on the #3 connector.
90.8 with key on, engine off, is a diganostic, indicating electronic problems, I suspect. Steve Brotherton will probably know, but it may be computer time (OUCH). If the computer is buggered, no way you are going to get the mixture set correctly without a CO meter, and you must get to the exhaust in front of the cat....
Can you check the EHA current? This will tell you if the system is working at all -- 0mA means there is no control current being produced.
Check the OVP relay if you haven't -- is you ABS or SRS light coming on and off while you dirve?
Also, check for a good ground for the fuel computer -- if it isn't grounded correctly, it isn't going to work worth diddly.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!