Hi all,

SOLVED: Ended up being an exhaust leak. more info in last comment

1998 Cherokee 4.0 with 246,xxx, NON-CA emissions
I have a problem I really need some help with.

Just over a year ago I had an issue where the jeep would lurch and stumble, lose power, and then backfire, sometimes 3-4 times in a row, pretty intermittently, It would also have this nasty smell each time that happened, and sometimes it would lurch and buck without actually backfiring.

I replaced the front o2 sensor with a bosch, no change. I brought it to a mechanic and he identified the problem as the o2 heater circuit fuse. He also said it had been backfiring through the intake (lean mix?). Replaced the fuse, and the jeep ran great for a year.

Then, about 3 months ago, I had it randomly do the same exact thing: lug, lose power, and backfire. No fuse blown, and only happened once. I ignored it at the time as a fluke.

Then, starting about a month ago, the engine started to buck/stumble, quickly drop and regain rpm, intermittently, at mid range RPM, and sometimes smell really bad (not gas, not rotten egg, don't know how to describe the smell except really nauseating bad exhaust fumes). Sometimes it would do it really bad and repeatedly, and other times it was subtle and just barely noticeable. Cruising at 50-60, around 1800-3000rpm is when I usually notice it.

I ran the jeep for codes, nothing. I found that the heater circuit fuse was once again blown, so I replaced that. I went under the jeep and found that the wiring for the rear o2 had been torn by the driveshaft. I replaced that wiring.

I tested the bosch front o2 sensor that I replaced a year ago and found it to be faulty, so I replaced it with an NTK. Symptoms did not change.

I refreshed the o2 sensor ground on the coil stud, and verified the continuity of all the wiring with a multimeter (see diagram, everything highlighted is what I verified to have continuity).


I also verified "sensor ground" in the diagram above at c1.

I went and ran codes again and this time I got
P0132 o2 circuit high voltage (bank 1, sensor 1)
P0138 o2 circuit high voltage (bank 1, sensor 2)


I did drive for a bit with the rear o2 unplugged, so this may have triggered the codes, or maybe they just triggered. I am very confused why this is the first time I've seen them.

I tested for shorts between the tn/wt and bk/dg o2 sensor signal wires and everything else in the harness and found nothing.

I verified that the heater circuits on both 02 sensors were within spec.

I tested and confirmed that the ASD relay was working properly.

I reset the codes by unplugging the battery, drove it for a bit, and tested again. This time I only had P0138 (rear o2 sensor high voltage). I also had p1694 (No ccd message from PCM to transmission) but I think this was triggered when I had some of the harnesses unplugged from the computer and had the key in the on position for a while.

I reset the codes again and drove it for a while, and the 02 sensor codes went away (lugging, lurching loss of power did not), and now I only have p1694.

That's where I'm at now, and I have no idea where to go next. I have tested everything I can think of and would really appreciate some insight from CF!!!
Thanks a lot, xjnm

 

Bump, anyone have any ideas? I'm going to try to research some more about other things that make o2 sensor-type symptoms. Would definitely appreciate anyone who can point me at more things to test. Also, I have heard that battery temp sensor can make it run lean, any thoughts on that?

 

On other engines I've had a manifold to head exhaust leak act like a broken O2 stuck lean. Between impulses it sucks air in, so "super lean" when really it is super rich, stinky, low power.

 

Thanks! I did replace the exhaust manifold, both manifold gaskets, and downpipe recently. I checked for exhaust leaks and didn't find any.

 

Update: Tested and confirmed that o2 heaters were getting 12v when the jeep was running. Any thoughts would be appreciated

 

And the O2 sensor output voltage swings or is it stuck around a certain value? If the sensor says lean and it stinks of raw fuel the sensor is probably lying since the exhaust leak was ruled out. If the sensor reads rich and it's rich I would look at coolant temp sensor. If the temp is stuck cold the ECU would keep fueling as if it were in "choke" warmup mode. I don't know about your year, but mine has two temp sensors, one for the gauge and one for the ECU.

Looking back at your details, I'm leaning toward intermittent wiring shorts or opens.

If it were mine at this point I would spend the $5 on an elm327 bluetooth OBD II adapter, and put the torque app on my phone and monitor live data for coolant temp, front O2 volts, tps and see what those sensors report during the failure event. I expect they will report normal data nearly all the time.

 

Thanks for the reply. I don't have an analog MM so I can't tell if the output voltage is swinging or not. It definitely is not running rich, I smell no gas smell at all, just the nasty exhaust smell.

I will check the temp sensors, I know one was replaced just over a year ago.

I will definitely buy that thing. I didn't know that existed. I will monitor some data and report back results. Thanks!

 

reading live data is the go, shows each O2 sensor switch on/off and at what voltage, short term fuel trim, open/closed loop, manifold vacuum etc

I have a BAFX unit, cost me about $30

 

I run the elm and torque in my Subaru all the time as an added instrument cluster. Sometimes I move it to a different car for testing for a little while. Much great data can be had with these things for not much money. OBD II and newer only. I had to get a special unit to read such data on my 88 Jeep for a great deal more money. still worth having for someone who does all their own work.

 

Update: Drove for about 100 miles with the o2 sensors unplugged and never had the issue pop up, so I am confident to conclude that means it is o2 sensor related. The elm adapter is coming tomorrow so I should have data by wednesday. Any recommendations on torque lite vs torque pro?

 

I have the pro version, but I didn't really notice what was 'unlocked' by the upgrade. I'm sure the free version would get you everything you need.

 

Drove it 30 minutes to work today and the issue did not pop up (O2s plugged in now). I recorded data while cruising at 60mph 2000rpm where I usually have the stumble. Video below, no need to watch the whole thing.

I recorded:
Short term fuel trim bank 1
fuel trim from upstream sensor
fuel status
downstream sensor voltage
long term fuel trim
upstream sensor voltage

Neither o2 sensor seems to be swinging back and forth from 0v to .9v correctly, but I wonder if this is a software issue rather than an issue with the sensors. My elm adapter and app combo seems to be kind of sketchy at times
Any thoughts appreciated.

 

Solved!

I went and double checked for exhaust leaks and found that the bolts from the manifold to the downpipe were slightly loose. Since tightening these, I have not had it stumble or backfire once. Hope this helps anyone with the same problem!