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4.0L running rough and rich when cold - 89 MJ


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it does smooth out eventually but thats still wrong. I am going to replace the O2 sensor cuz it appears to be OEM and the plugs/wires/cap/rotor are new. The MAP sensor is hooked up and the vacuum lines intact but I unplugged the O2 sensor from my 89 XJ once and it pretty much did nothing so I am guessing I have something else going on. Maybe the ECM or one of the dozen other sensors but I could use some input on this one.

Thanks in advance.

Rick in CO

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The O2 sensor does nothing during warm-up. On initial start-up the Renix system runs in "open loop mode," which basically ignores sensor input and uses a rich-bias, pre-mapped air-fuel curve. I'm pretty sure the O2 sensor is one of the sensors that gets bypassed because, in my '88 Cherokee, on cold mornings I can feel a stumble followed by a jump at the point when it seems to be about the right temperature to be switching into "closed loop mode."

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The O2 sensor does nothing during warm-up. On initial start-up the Renix system runs in "open loop mode," which basically ignores sensor input and uses a rich-bias, pre-mapped air-fuel curve. I'm pretty sure the O2 sensor is one of the sensors that gets bypassed because, in my '88 Cherokee, on cold mornings I can feel a stumble followed by a jump at the point when it seems to be about the right temperature to be switching into "closed loop mode."

 

That's why I like having an A/F meter installed. Even an el-cheapo narrow band does a good job of monitoring the open-to-closed loop shift, and lets you know how the O2 sensor is performing. Well worth having IMHO. :cheers:

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