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Old 05-18-2015, 07:18 PM
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1998 cherokee stuttering after start-up

Hey everyone new to the forum. Just started having issues with my 98 cherokee 4.0l I bought recently and was hoping someone might know what the heck is going on.
So whenever I start my jeep up after it sitting for a little while(10+min) the engine will momentarily stutter and act like it's being starved of fuel. I suspected o2 sensors as I had recently cleared some high voltage and heater circuit codes that came up. Ended up replacing both. Didn't fix it. I hooked my code reader up and watched the o2 sensors to see what they were doing. I noticed something interesting. So on start-up, the upstream o2 sensor reads 1.0 volts. After about 10-20 seconds of driving, the o2 sensor seems to kick in but when it does it gives a reading over 1.0 volt, anywhere from 1.2 to 1.4 volts. That's exactly when the engine stutters. All I have to do to stop the stuttering is ease off the throttle. After that it won't stutter at all until it cools down. I'm guessing the o2 sensor is momentarily giving a rich reading so the pcm cuts the fuel to lean. Is that correct? Is it normal for the sensor to give a high reading right off the bat? I thought the pcm was supposed to take over until the o2 sensor was hot? Is the o2 sensor maybe taking too long to warm up, possibly a bad wiring harness? I tried driving with it unplugged and it drives fine and doesn't stutter at all. I should mention that I did reset the pcm to clear the previous codes, but it ran fine and didn't start stuttering until a while after.
Just wondering what else I could test. Also, the previous owner cut the exhaust to the cat and gutted it. Could this affect the upstream o2 sensor? Thanks in advance

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Old 05-18-2015, 09:42 PM
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Re: 1998 cherokee stuttering after start-up

Welcome to the garage.
When the engine (and consequently the O2 sensors) is cold, the fuel control works in open loop mode and the PCM is not using the O2 sensor readings. Fuel injection will be done according to a pre programmed fuel schedule in the PCM. When warmed up, the fuel control will work in closed loop mode and now the O2 sensor readings will be used by the PCM to trim the fuel, so that an as close as possible air/fuel ratio of 14.7 to 1 will be reached.
The O2 sensor reading can be max 1.0 V (when engine is off), but during normal operation it should go in between 0.2 V and 0.8 V. A voltage above 1 V is not normal and points towards a short somewhere in the O2 sensor wires, or in a connector. I would check the O2 sensor wires for shaving and the connectors in the O2 sensor wiring until the PCM for moisture or corrosion. Normally you should get a DTC set for O2 sensor voltage too high.
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Old 05-18-2015, 10:03 PM
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Re: 1998 cherokee stuttering after start-up

Ok so figured it out and I'm kicking myself in the arse for not checking this. So turns out there must have been a short a while back because I just checked the fuses and what do you know a 15amp was blown. Replaced it and viola fixed. I took a closer look at the old downstream sensor wire and there was a tiny spot underneath the wire sheathing that that looked like it rubbed on the driveshaft( not sure how that's even possible). Ridiculous. Oh well live and learn I guess lol and thanks for explaining that frango
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Old 05-19-2015, 08:34 AM
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Re: 1998 cherokee stuttering after start-up

Which fuse was blown? If it was for the O2 heater, then its strange that the PCM didnīt see it.
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