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2000 STFTs stuck at 98% ??


NAXJA Forum User
Hi all,

My son has a 2000 XJ 4.0 that runs fine, but sometimes rough. I've heard all XJs idle a little rough, but I wanted to make sure that this isn't costing him a bunch in gas, so I fired up the Piston app and started taking readings. Need to verify what I've found.

Overall STFT and LTFT reported for each bank is within the normal range. Bank 1 gets an LTFT of -1.6% and Bank 2 gets 3.1. So different, but not "abnormal."

The pre-cat O2 sensors on both banks seem to be OK. Their frequency oscillates, and even though the waveform is not terribly clean, it isn't flat. The STFT for these is within the normal range.

The post-cat O2 sensors on both banks seem to be OK. Their frequency oscillates, and even though the waveform is not terribly clean, it isn't flat. Whats weird about the post-cat sensors is that the ECM is reporting a flat STFT of 98.2% for both.

I've been reading a bunch and trying to figure out if this matters and could indicate that these O2 sensors are bad. But, it seems more likely that the ECM is just not taking the STFT data into account for the post-cat sensors, and therefore they should be fine. Thoughts?

A little bit more info since I know its going to be asked:
  • The head isn't cracked. The original one did and I rebuilt with a good TUPY head last July.
  • Spark plugs were all replaced when the head was replaced. The coil was not replaced, but seems like that should be fine.
  • TB was also cleaned when the head was replaced. IAC and all that should be good.
  • Engine idles a little bit rough and even misses occasionally even at op temp. It does idle at the correct RPM, but the tach can jump a bit and varies RPMs by maybe +/- 40 or so.
  • I can hear that he does have an exhaust leak somewhere, but its not a bad one (and idk where it is). Engine starts and runs totally fine, has ample power, etc. Its not much more than an annoying sound. He had had his tailpipe break in front of the muffler at one point which caused a HUGE difference in idle and performance. So, I can see where that might be contributing, but it can't be that much in my mind.

Think my downstream O2 sensors are bad?
I would confirm the data with a known good scanner before you make any hard decisions!
I would confirm the data with a known good scanner before you make any hard decisions!
That's a good idea. I'll take that advice. However, assuming I get the same results, what is your inclination?

Sent from my SM-G998U using Tapatalk
Fuel trim data from Sensor 2 (post-cat) is typically ignored. Bank 2 sensors are used for Cat health and not fuel adjustment.

Here's a support response from an OBD Scanner manufacturer


This is the correct answer. Post-cat O2 sensors, at least on older vehicles, are used to monitor cat efficiency and aren't used to control AFR.

It sounds like you are also monitoring the O2 sensor voltages which is what you'll want to observe. If the post-cat O2 sensor voltages are varying, and the ECU is not indicating a cat converter issue, then all is ok with your cats and the rear O2 sensor.

The pre-cat O2 sensor voltage will continuously oscillate during closed loop conditions which is typically during relatively light engine load conditions such as idle and cruising at light throttle. During idle, it should oscillate back and forth fairly slowly. The oscillation frequency will increase as the air flow rate through the motor increases. Data communication speeds between OBD-II devices and the ECU are fairly slow, and you are probably not seeing the full O2 sensor voltage waveform. The OBD-II device will track O2 sensor voltage better if you disable monitoring of all other items.

As far as idle quality goes, I would say that a smooth idle is entirely possible. There is nothing rough about the idle quality of my 2000 XJ. Never misses at idle. I know people say that copper tip plugs are the way to go on the 4L, but I run double iridium, and I tossed my 21 year old stock coil pack for the Accel high performance coil pack (which costs only a few bucks more than a quality OEM NTK replacement). The front O2 sensor on my XJ went bad shortly after I bought it in 2020, so I also replaced the front and rear O2 sensors at the same time with NTK sensors.

The muffler of my 2000 XJ has a small water drain hole where some exhaust noise leaks out. However, if the exhaust leak is in the engine bay, then there is something else going on.
I dont believe Accel makes a coil rail but MSD does but because of the cost you would be better off going to spark plug wires and a Screamin Demon coil.
BTW, for even more performance and horsepower I added a MSD igntion box to mine.

Having spark plug wires has so many advantages when it comes to checking/reading your plugs when tuning!