99 TJ.....engine issue on warm up

[Bnine]

O2 sensors are not operational during warm up (open loop). Cmon guys, you know better then that.

Tap the map with a screw driver during warm up and see if anything changes.

[birdo]

Well......took a long-shot and went and saw Jay on Thursday. We poked and prodded, swapped PCM, did OBD checks, scratched our heads and looked up at the sky a lot. Jay suggested I might try replacing the pre-Cat O2 sensor. Friday, and with a lot of swearing wrenching and penetrating, oil I got the old sensor out and replaced it with a new one.........I might be jumping the gun here, but I did a lot of stop and start errands Friday and so far so good...no belching, farting, bucking and other non-sense.  Maybe....just maybe we finally got it fixed. O2 sensor......never threw a code.....wow!

[BrokenAxle]

Glad you got it sorted out!

[Bnine]

Well......took a long-shot and went and saw Jay on Thursday. We poked and prodded, swapped PCM, did OBD checks, scratched our heads and looked up at the sky a lot. Jay suggested I might try replacing the pre-Cat O2 sensor. Friday, and with a lot of swearing wrenching and penetrating, oil I got the old sensor out and replaced it with a new one.........I might be jumping the gun here, but I did a lot of stop and start errands Friday and so far so good...no belching, farting, bucking and other non-sense.  Maybe....just maybe we finally got it fixed. O2 sensor......never threw a code.....wow!

Hopefully its worked out for you. Still makes no sense though.

Either your description of the problem was wrong, or the computer you swapped did something.

An o2 sensor should never effect the running of a vehicle during open loop. Its virtually impossible unless there's faulty programing involved.

[vantagetes]

Hopefully its worked out for you. Still makes no sense though.

Either your description of the problem was wrong, or the computer you swapped did something.

An o2 sensor should never effect the running of a vehicle during open loop. Its virtually impossible unless there's faulty programing involved.

This is Chrysler we are talking about... 

[birdo]

I think you guys will find this interesting...it's a primer on heated O2 sensors. It might dispel a few myths about these gizmos.

http://www.picoauto.com/applications/lambda-sensor.html

The section on "Heated Oxygen Sensors" is very interesting.

Why my failed sensor never threw any codes remains a mystery.

[Bnine]

Yeah interesting enough, but still doesnt explain why a sensor that isnt active during warm gave you issues..................during warm up.

All that article explains is how 02's work, and what the symptoms are when they are failing. Again, all closed loop diagnostics.

The fact that your symptoms were exhibited during warm up (open loop) still indicates that it should not be 02 sensor related. Im pretty sure you inadvertantly fixed while you were messing around with everything else.

Either that, or its possibly an intermittent issue thats gone away for the time being.

[TJ Sparky]

  Not too sure about the 'closed loop' aspect of it but this is common occurance with Jeeps that have the upstream O2 sensor. I had the same problem and received the same advice on JeepForum.com. There are literally tons of posts/threads on it all have the same outcome; ' if you're having issues on start-up first unplug the O2 sensor and if the problem goes away the sensor needs to be replaced'..........fixed mine too.

[birdo]

I have to rule out any tinkering we did. There is no doubt in my mind that the O2 sensor was the culprit. When I drove over to the parts place to get the sensor, the Jeep was still bucking and snorting on the short ride over. On the way home (approx 30 kms) as soon as the engine got up to heat, it ran fine. Same old pattern. The next morning I replaced the O2....fired it up and not a burp, hiccup or misfire. Been fine ever since.

All I can tell you is that if your Jeep starts to misbehave, as mine did, consider your upstream O2...especially if the one that's in there has been there for a while. Yes it should throw codes, but it may not always do so. 

[Bnine]

As you can tell this has bugged the crap out of me.

Here is the explaination of whats happening.

http://www.justanswer.com/jeep/3vwgi-misfiring-brief-period-during-warmup-driving-maybe.html

The sensor was acting up after your jeeps were "warm" and went into closed loop.

By the way, according to these guys you should disco the batt for a while and clear the fuel mapping memory for the best results when you change that sensor.

Oh, and that also explains why it never threw a code. The voltage remains within acceptable range, its just slow to bleed down. It will only throw a code if the voltage spikes outside of the operating range, one way or another.

[LEMON-AID JAY]

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