Re: <e30> Oxygen Sensor: Problem, Test Results, Help With Diagnosis Please

[Chris Baisley <cbaisley@xxxxxxxxxxxxx>]

>From: Jason Dougherty <skeptic@domain.elided>
>Subject: Re: <e30> Oxygen Sensor: Problem, Test Results, Help With
Diagnosis Please
>>Test: voltage output of oxygen sensor, warm engine running at idle: varies
>>from 0.08 to 0.55 volts
>>Test: voltage output of oxygen sensor, warm engine running at idle, oil
>>filler cap OFF: drops to 0.05 volts steady
>>Conclusion: oxygen sensor OK
>
>Does the Bentley manual say that this is indicative of an OK sensor?  It
>doesn't sound quite right to me.  Normally, O2 sensor output varies from
>0.0 to 1.0 volts (0.0 = lean, 1.0 = rich).  When everything's warm, the
>output voltage should be varying back and forth between these limits.
>The voltage you measured sounds low.

Bentley doesn't say what the range should be, only that it should be
constantly changing, which it does.  Since the voltage drops to almost zero
in lean condition (oil filler cap off), and the voltage varies at idle
(presumably neither lean nor rich condition), the test doesn't measure
output under rich (acceleration) conditions.  Which makes Jason's suggestion:

>I think the sensor may be dying and can no longer output the full voltage
>range.  You didn't say the mileage on the
>car, but this is a fairly common failure mode for an old sensor (>100K
>miles).  The ECU probably tests the sensor by looking for a voltage near
>1.0 under conditions where the engine should be running rich (acceleration,
>heavy load).  The sensor can't output 1.0 any more, so you get a "check
>engine" light.

a reasonable one.  Is there any way to test the sensor with the car parked
under rich conditions?

BTW, for those following this thread, the problem seems to have disappeared
since connecting and unconnecting the o2 sensor a few times to do the
tests!  Dirty contact syndrome?



Chris Baisley
Vancouver, B.C.
1989 e30 325i Bronzite
http://www.infomatch.com/~cbaisley/bmw.htm

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