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

[Chris Baisley <cbaisley@xxxxxxxxxxxxx>]

Original Symptom:

At highway speeds under light acceleration, "CHECK ENGINE" lit up.  This
happened very infrequently.

Latest Symptom:

At highway speeds under heavier engine loads such as uphill grades, "CHECK
ENGINE" lights up.  Light goes out if I lift off the throttle, comes back
on when I accelerate.

DME Code Readout:

1221, which indicates an oxygen sensor problem.  I have checked this
several times throughout the duration of my problem, and the DME always and
only reads out code 1221. 

	Aside: How to do this?
	http://www.infomatch.com/~cbaisley/bmw2.htm
http://www.eskimo.com/~dalus/bmw/repair_faqs/motronic.html

Testing (as per Bentley Manual)

Test: Voltage to oxygen sensor heater with engine running: 13.46 volts
Conclusion: heater power supply is OK

Test: resistance of oxygen sensor heater (engine cold): 31 ohms
Conclusion: could be a problem, Bentley says should be less than 5 ohms,
but engine is cold so test probably not valid

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

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.

Comments and Questions

So, it would appear that the heater portion of my sensor is the problem, as
everything else checks out OK.  What makes me scratch my head is that the
occurrence of the problem (ie. CHECK ENGINE light ON) is not random, it
occurs predictably now under heavy engine load at highway speeds.  I would
expect a faulty heater to cause problems either constantly, or randomly.

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).

Thanks for your help!


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

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