misfire~stumble....solved - JeepForum.com
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post #1 of 4 Old 04-24-2015, 07:27 PM Thread Starter
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1998 ZJ 
Join Date: Feb 2012
Location: Wilson
Posts: 622

Symptoms: misfire, stumble, intake backfire at idle or under load, only during engine warmup. No CEL.
Replaced plugs, wires, cap, rotor, CPS, and TPS. No improvement. Did wiggle tests at all computer and electrical connectors. Swapped multiple parts with known good ones. Countless hours connected to various scanners. O2 sensors operating within specified parameters.

After fretting over this for a couple of years, kept digging. Finally, after reading stuff here and on other sites, decided, just for giggles, to unplug the upstream O2 sensor. No stumbles or misfires over the past 24 hours, will continue to monitor over the weekend, but suspect that the heater circuit was borderline, not bad enough to trip the CEL but enough to cause the symptoms until the exhaust gas heated it enough to compensate. New NTK brand sensor located and priced, hoping this is it. Just Empty Every Pocket.

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post #2 of 4 Old 04-24-2015, 07:38 PM
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1997 ZJ 
Join Date: Dec 2008
Posts: 8,580
Glad you finally figured it out. I think the o2 heater circuit sometimes pulls too much current and causes low current to the the other components on the ASD circuit.

Remember that the o2 heaters are grounded on the block, so if you haven't cleaned those, it would be a good idea to do it when you get a round tuit.

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post #3 of 4 Old 10-08-2015, 06:33 AM Thread Starter
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1998 ZJ 
Join Date: Feb 2012
Location: Wilson
Posts: 622
Just to follow up: I finally got "A Round Tuit" Installed new NTK sensor, light is off and staying off. Of course, this being a Jeep, on the way home it threw a P0720 code........... I checked the harness for the output speed sensor, all seemed good. Installed a new sensor, 15 bucks, and that seems to be okay so far, 60 miles and light has stayed off..... Of course, now my temp guage is all over the place, and the ABS is misbehaving. BUT THAT DARN LIGHT IS OFF!!

Retreat? Hell, we just got here......
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post #4 of 4 Old 10-12-2015, 05:50 AM
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1998 ZJ 
Join Date: Aug 2005
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I went through this EXACT issue. NO CEL, nothing. Drove me nuts for months. Finally, via suggestion on this forum, I unplugged the upstream O2 sensor and it ran great. Installed a new NTK/NGK a few months ago and it's still running strong.

2001 Patriot Blue WJ 4.0 |1996 Charcoal Gold ZJ 4.0 | 1998 Stone White ZJ 5.9
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