Could be O2 or ECT Sensor making it run rich (archive)

Henry, mine was running 2.1-2.4 in P or N and 2.7-3.0 in D...sounds like yours is high by nearly 50% assuming all else being equal.
As for the O2 sensors, there are a few things you can do to check them out before having to go to the trouble of having to use an exhaust gas analyser:
1) the ones we have are called "two-state heated lambda sensors". Lambda is the excess air factor where 1.0 indicates a perfect stoichiometric mix and >1 indicates lean (excess air) and <1 is rich. The sensor's voltage and internal resistance are both sensitive to temperature and are not reliable until temperature get up to about 350�C and are therefore heated. If the heater circuit isn't working you will have poor results.
- check fuse #28 is good (easiest)
- check for 12V output from the O2 relay (Aux. Relay Panel RH side) - it is the orange?yellow? single one in with the two DME relays (white) and fuel pump relays (black). The green wire with white tracer powers the heaters, make sure you have juice there. (next easiest)
- check for 12V at the connector between green (#4) and brown (#3) wires. This is the hardest to do because the connector is under the transmission. Personally, I would check the sensor output at the ECM connector before I would crawl under.
2) they are called "two-state" because they have a characteristic radical transition in voltage output as the mixture passes through ideal (1.0). Their output voltage is about 900mV (0.9V) at slightly rich mixtures and it flips to about 100mV (0.1V) at slightly lean mixtures. Warmed up at idle, you should see the output from these sensors fluctuate back and forth between about 200mV and 800mV (0.2V to 0.8V).
- you should (must) use a high input impedence multimeter for this (almost any digital one will do)
- you can check for these voltages either at the connector (again, hard to get at under transmission) by connecting to the yellow (#1) and black (#2) wires, or
- at the input connector to the two ECM computors (my choice). The yellow and black wires appear at location #10 and #28 respectively in the connector, you want the black one #28. The ECM's are located in the main E-box (RH side firewall) - from firewall to front the modules are ECM #1, EML, ECM #2 and ABS/ASC. With ignition off, undo the screws holding the wiring harnesses to the ECM's and remove the little cover. Remove the single screw at the front of the main cover, raise the latch handle and lift the black cover off the top of the connector. You will see numbering for each connection up one side and down the other of the connector. Locate #28 with the black wire and insert the multimeter probe in with it. Start the engine and let it idle. You should start to see results within a few seconds.
If one of your sensors has failed and the result is the engine running rich, the sensor would have to be giving the ECM a false lean signal (i.e. ECM trying to richen up to get exhaust back to 1.0) therefore you would see an unusually low output signal from the O2 sensor...likely hanging around the 200mV to 400mV range.

Now after going through all of that, one other thing that artificially sets the mixture rich is the cold start sequence which is largely based upon the engine coolant temperature (ECT) sensor. You might want to take a look there as well. A subject for another post...!

Good luck...hope this helps,
'88 750iL

I went and looked at your old posting on checking fule usage by doing the temp button and pushing 1000 and 10 and then the 1 three times. Worked like a charm! So here are my numbers: At 72 Degrees it's running at 3.1-3.4.
So my question is what is the norm for a V12? At what number do you suspect the O2 sensor, etc....
Thanks in advance!
Henry
89 750iL 170K

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