jburg318

Wanted to give a background and get people's thoughts.

So It was raining cats and dogs for days where I live and I had my Cherokee outside parked in the rain. The rain let up and I took the XJ to work. About 5 or so miles from home, I was cruising at 35 to 40 mph and the vehicle just lost power, rpms dropped to idle, and she started to buck. Kind of felt like the transmission was slipping, but that wouldn't make sense with the rpms dropping in my mind. Eventually, I felt like I was able to limp to work, but I got nervous and drove her home to park her. On the way back home, she felt like she was about to have the same symptom again, but it was maybe a split second.

Today, I scanned the engine, and I have a P0138 code and the sensor is pinned at 1V and 99% O2 (the latter of which is impossible since the atmosphere is 80%), so its definitely shot. Maybe, there is a bad wire somewhere and the sensor shorted from the rain? Just a running hypothesis, but I am picking up an NTK today to replace it. Also, when I started her up and let her warm-up for a bit today, she was definitely running a little rougher than usual. As a note, the upstream sensor voltage signal is bouncing around, so that appears to be functioning. From my understanding the upstream O2 sensor regulates the fuel trim to a greater extent.

My question is, has anyone ever seen this symptom before from the downstream O2 sensor? I have read everything from a Crank Position Sensor, Cam Position Sensor, upstream O2 Sensor, MAP sensor, TPS sensor, engine grounds, and need for a tune-up could cause similar symptoms. I have not yet seen someone say downstream however.

I guess I can just replace the downstream and see if the symptom comes back. It seems like a lot of people spend hundreds on sensors and never really actually pin down what the problem is.

Thanks!

BlueRidgeMark

Downstream? Behind the cat? I don't have the FSM for that year, but at least for 97 (and I think, 98) back, the downstream is not part of running the engine. It only exists to tell the PCM when the cat is shot.

I don't think the downstream sensor is the problem with the way it's running. I'm not saying it's okay, just that it's not causing your problem.

UNLESS the later models DO use the downstream, but I very much doubt that.

jburg318

Yes, the O2 sensor after the cat.

After reading a bit more due to your post, it appears the downstream sensor not only functions to check the catalytic convertor, but also as a reference for the upstream O2 sensor.

So if its pinned, it may affect the drive ability of the vehicle if the downstream voltage is out of range.

Another user added that the FSM is extremely clear that "all O2 sensors are used when calculating injector pulse width." He did say that the FSM is vague for whether or not the downstream O2 sensor is used for setting trim for 97 to 99.

Everyone local was out of the NTK so I am ordering from RockAuto. I am also going to pull apart the throttle body and give that and the IAC valve a thorough cleaning.

I can run my scanner to check for intake manifold pressure as well to see how the MAP sensor is doing.

The vehicle looks to have the original cat, so it may be that as well as other have suggested that as a potential cause. I will have to see if I can get real time emission reads as well. I was going to replace the entirety of the exhaust and get a custom downpipe with a flex joint fabricated and installed, may have to do it sooner rather than later.

s346k

as with 99% of every other obd2 vehicles - the downstream o2 has no effect on the engine. it’s just a tattle tale. mine has been disconnected for almost 100k miles with 0 i’ll effect.

id be looking at sensors that affect how the engine runs, like the others you mentioned.

BlueRidgeMark

Did you read that from an official service manual? OR some forum member's opinion?

Unless the 98 is different, the downstream has nothing to do with the upstream.

jburg318

It was a blurb from Dodge and I beleive

I went back and read the 97 FSM and there are two sections on O2S function with a specific section describing it to check the catalytic convertor efficiency.

I ran the vehicle yesterday with my scanner hooked up and luckily the problem popped up again. While real-time monitoring the upstream (pre-cat) O2S and the intake manifold pressure via the MAP the problem showed up.

While idling, the intake pressure began to climb for only a few seconds, and the upstream O2S got pinned to 1 volt shortly after and the vehicle started to run like crap. I really think the MAP sensor is on its way out and the engine is running rich to compensate the increase in absolute pressure. Either that or its an intermittent vacuum leak, but I feel like that is less likely.

I will take a look at the fuel trim and see if I can get it to do the same thing again.

BlueRidgeMark

This is not a Dodge. It is a Jeep. Just because Chrysler owns both, does not mean that they operate the same. There are many commonalities, and there are many differences. If you didn't see it from a Jeep manual, you should disregard it.

Yep. That's what it does. It is not used to regulate the engine.


I think you are probably on the right track here.

jburg318

I just hope its something that I am not pounding my head against the wall for the coming months.

I am now thinking the reason it threw the downstream O2S code is that the upstream got pinned in response to the MAP sensor/increase in absolute intake pressure and since the downstream O2S signal response is delayed, it followed suit, but got trapped for a longer period of time in the cycle?

I was reading that not all sensors throw a code when a problem occurs and sometimes the code is result of the actual problem because of how much time/recurring instances an error has to occur to turn on the MIL.

The FSM does talk about what it takes to trip the MIL for different sensors.

BlueRidgeMark

Yes, the codes are clues, but they don't tell the whole story. I don't know for sure, but I think your theory seems reasonable.

jburg318

Alright, after my Jeep sat for a few weeks, I was finally able to get to it yesterday. A lot of people never follow-up with the solution to their problem, so I wanted to provide a detailed account.

Symptom: Bucking and loss of power when driving. Seemed to get worse over time as I was starting her every few days to a week to make sure battery was staying good. Noticed during this time vehicle would sputter and about to stall once getting to operating temperature. Long-term Fuel Trim (LTFT) extremely rich at around -14 to 15% and exhaust was strong.

After reading through the forums, it literally sounded like it could be anything from a failing cam position sensor, to MAP, to fuel regulator. Being one not to just throw money at the problem, I took a systematic approach.

Rich condition could be the result of one of the following (plus more) issues:
Bad fuel pump regulator - Spec. 49.2 ± 5 psi after vehicle has warmed
Bad or leaking injector(s) - Spec. 12 ± 1.2 ohms at 20 ºC (around room temp)
Bad spark plug(s) and or wire(s)
Clogged catalytic convertor or muffler - PSI > 1.5 at idle
Bad upstream O2 sensor
Bad compression (already saving for a stroker build, but she would sit until around December for this)
Upstream exhaust leak
etc.

Fuel pressure test and results
So I rented a fuel pressure gauge from AutoZone warmed the the engine to operating temp and hooked the gauge up to the Schrader valve and starter her back up. Pressure was pinned between 48 and 50 at idle on the gauge so the regulator is good to go (thankfully).

Fuel injector test and results
Did the screwdriver test on the each of the injectors and they all sounded like they were clicking and firing just fine, this would suggest they are operating and getting power. Next, disconnected each injector and tested with a volt meter. All read between 13.2 and 13.4 ohms. Although not specifically within spec, the engine was a bit warm which should lead to a higher resistance. I figured they were alright since they were close to one another. Next moved to a spark plugs, wires, cap and rotor.

Tune-up and results
First, I labeled my wires and then replaced the rotor and cap. The rotor was a little chewed up, but upon visual inspection there were no cracks on the cap or rotor. Replaced rotor with Standard Blue Streak Part No. FD316 and distributor with BWD Part No. C275. Rewired and started her back up, still running like crap so not rotor or distributor. Checked gap on set of new Champion 34122 plugs and started working my way from cylinder 1 to 6 replacing each plug by placing a dab antiseeze on each and torquing to 30 ft lb spec and each wire with a new Denso Part No. 6716128 matching length after putting a bit of dielectric grease on with a Q-tip in the inside of each boot.

Finally take the ignition wire off and go to place her on the ground... metal tab on the ignition coil side falls out of the boot onto the ground and clinks... M'fer. At that point I was pretty sure I found the problem. Replaced the last wire and started her up and she was running pretty good at that point. Drove her around for about 5 miles and no bucking or hesitation.

The wires on the engine from the PO were AutoLite, which I have now come to learn are crap.

Truck ran pretty good back and forth to work today (about 6 or 7 miles each way) and I put some fresh gas in her. LTFT is now -3 to -4% in closed loop. Still seems to be a little on the rich side, but the motor has some miles on her, so I am not too concerned.

I am thinking the rich exhaust somehow fouled the downstream sensor now, I bought a replacement just in case and in the coming weeks I am planning on doing a complete exhaust swap including header (has a slight crack at cylinder 3) and I want the 4.0 to breathe a little better.

I should also note, I still have a stored code for P0138 in my memory that won't clear. Just have to get the computer to reset with miles I guess.