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

[Jason Dougherty <skeptic@xxxxxxxxxxxxxxx>]

Chris Baisley wrote:

>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.  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.

>Test: resistance of oxygen sensor heater (engine warm): varying from 5.1
>ohms up to 10 ohms
>Conclusion: heater element is out of spec (should be less than 5 ohms),
>probable cause of problem.

I doubt this is the cause of the light, but it's still too high.

>Guidance and comments would be appreciated, before I drop $300 to $400 CDN
>(ouch!) on a new four wire sensor.  Are there other tests that I (or the
>dealer) could do prior to replacing the part?  Anybody know a cheaper
>source for the part in Canada?  (Bav Auto has 'em for $124.95 USD, which is
>probably better even with exchange to CDN and duty).

You may be able to get it cheaper if you can find out the manufacturer's
part number (probably a Bosch sensor), and find out what other cars it was
used on.  The same sensor may be cheaper as a "Volkswagen part" than as a
"BMW part".

Hope this helps,
	Jason

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