SurplusFan98
NAXJA Forum User
- Location
- LA
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:
Think my downstream O2 sensors are bad?
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?