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I have read more on the Bosch O2 sensor operation and have confirmation on what I had suspected. You recall in your earlier posts (see link), that one of the sensors operated 0.2-0.8V (rather slow) and one 0.4-0.6V (very slow). From what I have read in the Bosch material about the dynamic response of the sensor, well let me simply quote "An oxygen sensor exposed to excessively high temperatures for an extended period of time may start to react more slowly to changes in the air-fuel mixture. This leads to extended phase durations (periodicity) in the two-state control pattern." It shows some examples of phase durations with normal being in the 3 second range and two 'aged' sensors with phase durations of 8 and 11 seconds. This bears a strong resemblance to your situation.
Assuming this is correct, the sensor has been operating at too high of a temperature for some time...too lean, yet you indicate a high fuel consumption. You also indicated having replaced them twice already. Maybe the failed sensor is just one of the symptoms rather than the cause. Just thinking here but,
a) the bad sensor bank may have an air leak,
b) maybe the heat from the CAT running too rich toasts the sensor.
'88 750iL
...O2 sensors were replaced twice with no effect. O2 heating circuits OK as tested between fuse28 and ground (2.8 Ohm with both connected, 4.2 with one). O2 voltage
0.2-0.8V (rather slow)on one and 0.4-0.6 (v.slow) on the other...
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