Help with O2 sensor measurment interpretation requested.

["Smith, Kirby A" <kirby.a.smith@lmco.com>]

After tearing into my ECU locations to check their part numbers, I
investigated why my exhaust has become different in olfactory character.
The differential pressure control read 10 mA instead of the 0 mA that it
should be for my car (and was last summer).  Checking the temperature
sensors showed proper resistance.  Disconnecting the O2 sensor, the current
went to zero as it should.  Grounding the O2 pin (ECU side) led to 10 mA, as
it should.

OK, bring out 'scope, hook to disconnected O2 sensor, experiment.  Reading
is 175 mV, more or less, no matter what, more or less.  Putting propane into
the air filter intake had no effect on the reading, although the engine ran
slightly better.  Pulling the oil dipstick had no effect on the reading,
although the engine ran poorly to barely.   Shorting the lead from the ECU
to the O2 sensor to ground, which should richen the mixture as seen above,
had no effect on the O2 sensor voltage.  The only way I could stimulate it
was by revving the engine; there was a momentary jump to around a volt
during the throttle dump.  Transition time was 10 to 20 msec.  (Thanks Mike
Bywater for your useful submission to the list 5 Dec 97.)

So although some of these symtoms seem to be similar to those one would
expect from a vacuum leak, the dipstick test suggests not, so I suspect the
O2 sensor, which could have been contaminated by oil burning in hard corners
during our spirited run to Mt. Wash. last spring.   (There was an overfilled
crankcase, thanks to a helpful mechanic.)  Can any of you mavins confirm
that this behavio(u)r is characteristic of a bad O2 sensor?

                .... Kirby   (Kirby A. Smith)
                              2 x 1988 90q
                          New Hampshire USA