After the 2000 XJ (AW4) warms up the O2 sensors will occasionally stop working while decelerating, and if I decelerate long enough or enough times (basically if I drive 15+ minutes) the fuel trims stop happening. I know this because I check my scanner to see what's going on, and I know it's happening because I have a wideband AFR gauge to let me know what's happening, and once it's done it long enough it stays in the 17.5+ range and becomes impossible to drive. It will no longer accelerate and will sputter under throttling so I have to pull over and shut it off. Sometimes letting it sit for a minut and turning it back on resets whatever the problem is.

The wiring harnesses are fine, however the post-cat O2's haven't been hooked up since I had the engine rebuilt in February. 1 set of those 02 sensors and their associated fuel trims should be in the pictures enclosed below although I am not sure which set is which.

I would think the ones showing .84/.80 volts are the O2's used for the fuel trims, but who knows.

For whatever reason the ECU wants to stop making fuel trims after it's warmed up and been under the influence of braking. I'm not sure why that is. I've watched it during driving and there are fuel trims taking place up until it decides to stop working, but one is usually +25% while the other is -25%, and I suspect something is wrong there, and it's not a manifold vacuum leak because I was sure I didn't have one before and replaced the gasket anyways with a brand new one, and it solved nothing.

The vacuum is stable and I replaced the brake booster not too long ago because I thought that was the issue. I did legitimately have a bad brake booster, however this problem was there then as well (ECU would stop giving fuel trims and just go to flat 0's). I tested the new brake booster which held vacuum and replaced it. The previous BB would drop 10" of vacuum while decelerating when I pushed the pedal so I was certain that was the issue. When the new one went on it would drop 2-3 and my misfires went away; however it still only drops 2-3 and the misfires are back, so whatever it was originally, the brake booster was only aggravating the problem with low manifold vacuum.

I hope all that made sense.

 

So it appears 1/1 and 1/2 are both the Bank 1 sensors; 1 being the upstream and 2 being the down stream. So 1/1 and 2/1 are the upstream sensors, which are the .80 and .84 sensors, which are reading too rich (which should cause a negative fuel trim, but instead I have all 0's).


Edited to add California 4x O2 sensor diagram.

 

Whatever that is It's amazing. That is a tuners wet dream to have the adaptive learn turned off at will. What is the fuel system status open or closed?

 

+ or - 7-9%. The Banke 1 sensor needs replaced. It's an aftermarket Bosch that I don't trust and I see it's voltage constantly jumping from .2V to .84V. The OEM unit on there, or NTK, hovers around .43V to .57V.

Under consistent 2500 RPM's the bank 1 sensor is at a constant + or -25% while the rear O2 that's an NTK is the opposite (I forget which one is + or -, but they're ultimately 50% apart total which is way wrong).

I'm going to try to get the downstream O2's wired in tomorrow and plugged in and see if that clears it up, then if not I'll reset the PCM and see how long that lasts. Ultimately I'm certain I need to replace that front O2, I just don't see how it could be the cause of this particular problem.

I am getting the feeling that there is a PCM problem since the fuel trims just stop happening even though the O2 voltages are still oscillating. I can't see anything else that would cause that condition.

It's no wet dream for me because I can't drive it any more when it goes stupid. If tuners wanted no fuel trims they could just unplug the 02 sensors altogether. Electronics that are plugged in and giving readings while still not operating properly shouldn't be anyone's dream.

 

Coffeecommando - I think I know one of your issues cause I was having the same one while chasing an o2 issue. I thought mine was doing something similar. It ended up being the scan tool itself. While on the same screen you are on, the bank 1 and 2 sensor 1 was switching voltage super slow, sometimes to the point where you could barely tell it was changing. My scan tool has an option to graph instead of just show a number value. When in the graphing mode, you could see the voltage switching in real time as it should even though the main screen showed little to no movement.

So long story short, all those numbers should be going up and down in values rapidly. They dont just stay at a certain voltage.

 

The bank1 bosch sensor is doing that between .2 and .8V. From what I have read and watched that is what it's supposed to be doing.

The bank2 NTK sensor is stying right around the middle zone which is not how the O2 is supposed to operate. That was also the same sensor that got washed in BlueDevil Head Gasket Sealant because of a leaking head gasket a year ago. I cleaned it off and everything was fine.... for about a month. I'm wondering if it's time to replace it.

It seems like the ECU might be going into open loop, maybe the bad O2 sensor has something to do with it (it sees one operating correctly and the other one is not within normal parameters). It's in open loop when it starts up for about a minute while the O2's heat up and during that time the AFR's are lean and it has the low power. As soon as it goes closed loop the AFR's go down to 14.5-15 and it runs excellent.

The only thing I can think of at this point is it's going back into open loop at some point, and when it does the voltages are both over .8V which is supposed to be the upper limit of an operating O2.

The FSM doesn't show two pairs of O2's in the electrical diagram so I have to assume they're sharing a common ground and two separate positive switched leads from the O2. I imagine if one is off it's going to affect the other. I remember a retired Chrysler mechanic told me that years ago when I did my first engine rebuild and I never forgot it. Maybe now that information will prove to be true.

Edit: He was right. THis is from the 2000 FSM:

 

SOmething else I forgot to mention that might be a factor now that I am watching videos is that my vacuum is down the 13" when hot at idle, and cruises at around 20. It used to be 14-15 and idle and 22 at cruise.

I also have a P0455 for the Vac Pump Large Leak Detected, but I checked the vac lines with a gauge and hand pump a little over a month ago. That's when I identified the faulty brake booster and ordered a new one. All the lines held vacuum, surprising, except for the HVAC controls which very slowly bled.

I'm going to take my spare OEM down stream O2 sensors and replace the front ones with them and see if that clears up the issue. Finances for new parts are not in my favor at the moment.

 

Your scanner will report of you are open or closed loop

 

I completely forgot about that.

 

Note from DJ
Check this post out by CCKen.
Here are three diagrams of the 2000 2001 O2S circuit
https://www.cherokeeforum.com/f2/01-...iagram-169099/
See Post #4 by CCKen

 

Coffee - I think we are in the same boat now. Short term trims were normal for about 15 min. Then went 99% and didnt move. Do you have a very slightly rough idle Too?