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Re: If I unplug the O2 sensor,,,,
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 7 May 2004 06:33:00 Share Post by Email
In Reply to: Re: If I unplug the O2 sensor,,,,, JohnK, Thu, 6 May 2004 06:35:05
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I agree with John - unplug the O2 sensor, and the idle will become more stable. That's because the engine control will know the sensor is bad, and go to a default setting.

The O2 sensor doesn't set idle speed - it sets mixture for emissions reasons. The engine has to adjust fuel flow to keep the emissions in check and the idle in check. With just one to worry about, it's easier.

I disagree that if the O2 sensor signal isn't moving, then the sensor is bad. The sensor measure oxygen level in the exhaust, but the measurement range is very narrow. 0.5 volts is the perfect stochiometric ratio, 0.2 is lean, and 0.9 is rich. But not very lean and not very rich. To make the catalytic converter work, the ECU actually swings between slightly lean and slightly rich, about once per second. So the O2 sensor signal should swing between 0.2 and 0.9, crossing 0.5 about 1/sec. When the engine control sees a lean mixture, it'll add fuel. However, the O2 signal can adjust the fuel mixture only so much. For example, if there is a vacuum leak, the mixture will be lean. If the leak is big enough, the O2 sensor can't schedule enough extra fuel to make get the O2 signal 'off the peg.' So the O2 sensor signal will be reading lean, and it's telling the truth. Same thing for a very rich condition (e.g. bad fuel pressure regulator). Even a badly adjusted AMM can put the O2 sensor out of range. The ECU sees a pegged O2 sensor and produces a O2 sensor failure. It isn't necessarily a bad sensor, and many perfectly good sensors get replaced because of some other problem in the engine.



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