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I recently bought an 86 740 162k miles which the local ( we work on every make and model ) mechanic and eventually the owner gave up on.
Symptom: Runs rich at idle ( rich enough to load up and kill engine hot or cold ) but leans out and runs good at higher speeds.
The oxygen sensor seems to have fail but not completely. Starting cold the sensor in open loop produces about .1v at idle and at higher engine speeds
( higher temperatures ) increases to about .4v , if left to idle it drops back to .1v . It takes sustained higher engine speeds for sensor to go into closed loop with voltage fluctuations lazy and never more then .5v .
Note: sensor heater is good and has voltage.
I'm going to try out a Bosch 13 913 universal sensor and see if things settle down.
Tim
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