AFR Spiking at idle?
Thread Starter
Joined: Jun 2014
Posts: 1,241
Likes: 47
From: Noblesville, IN
AFR Spiking at idle? I've been chasing down an intermittent misfire on my stock LH6 in my 2007 Trailblazer, it is at times incredibly noticeable and will cause some stumbling off idle, or cause a significant shake in the car due to the sudden imbalance.
I started logging to see if I could determine if it was completely random or just specific to a bank or cylinder. Of course while logging it doesn't seem to do it as significantly (not enough for the logger to really notice) but I can still feel the shudder in the car.
What I noticed is that after idling for a short while, the AFR will spike up to 16.7 and hold there. This seems to be the cause (or is part of the cause) of the misfire, as it will eventually return to 14.7 and as soon as it does the idle RPM clears up and the car runs smoothly.
Does anyone have any idea why the computer would be commanding a lean AFR like this? Possible sensor going bad? Truck does have 200k on it.
I've attached a clip of a log showing the commanded AFR moving around (and the subsequent injector pulse following). If there's additional data points I need to log please let me know and I can get another run.
I started logging to see if I could determine if it was completely random or just specific to a bank or cylinder. Of course while logging it doesn't seem to do it as significantly (not enough for the logger to really notice) but I can still feel the shudder in the car.
What I noticed is that after idling for a short while, the AFR will spike up to 16.7 and hold there. This seems to be the cause (or is part of the cause) of the misfire, as it will eventually return to 14.7 and as soon as it does the idle RPM clears up and the car runs smoothly.
Does anyone have any idea why the computer would be commanding a lean AFR like this? Possible sensor going bad? Truck does have 200k on it.
I've attached a clip of a log showing the commanded AFR moving around (and the subsequent injector pulse following). If there's additional data points I need to log please let me know and I can get another run.
Last edited by HappySalesman; Jun 8, 2025 at 05:13 PM.
Thread Starter
Joined: Jun 2014
Posts: 1,241
Likes: 47
From: Noblesville, IN
I understand that, however I think you misunderstand me. I'm not talking about the actual AFR, I'm talking about the commanded AFR. If you look at the "TB AFR Stumble" log you can watch the commanded AFR randomly spike to 16.7 at 10.4 seconds in the log, hold there for 3.2 seconds, and then drop back down to normal. During this time the RPM's dip and become erratic, and O2's read super lean, but both before and after that happens the O2s read like I would normally expect.
What I'm trying to figure out is why the PCM would command such a lean AFR.
What I'm trying to figure out is why the PCM would command such a lean AFR.
I understand that, however I think you misunderstand me. I'm not talking about the actual AFR, I'm talking about the commanded AFR. If you look at the "TB AFR Stumble" log you can watch the commanded AFR randomly spike to 16.7 at 10.4 seconds in the log, hold there for 3.2 seconds, and then drop back down to normal. During this time the RPM's dip and become erratic, and O2's read super lean, but both before and after that happens the O2s read like I would normally expect.
What I'm trying to figure out is why the PCM would command such a lean AFR.
What I'm trying to figure out is why the PCM would command such a lean AFR.
Thread Starter
Joined: Jun 2014
Posts: 1,241
Likes: 47
From: Noblesville, IN
I'm just going to take a wild guess here that you installed off road pipes and removed the rear o2 sensors? The computers will normally lean the afr as a very quick cat efficiency test that you pretty much never notice. If it does not get the expected results, whether because there are no cats, or no rear sensors, etc., the test becomes more intrusive until it is noticeable. It can be turned off, but some software no longer allows access to do it. If you do still have good cats and rear sensors, I have no idea what is going on.
There was a couple of weeks late last year that I had to drive the truck with a leaky intake gasket that was causing an intermittent misfire. I suppose if the fuel was combusting on the cat it could have caused some damage but I've got no codes pending or otherwise.
Ill get some more logs with the rear o2s and see what theyre doing when the pcm leans the car out. If i can turn it off i might just do that and see if it solves my issue.
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Thread Starter
Joined: Jun 2014
Posts: 1,241
Likes: 47
From: Noblesville, IN
Got another log, this time with the rear O2s added in.
Fairly certain this is the computer doing a cat efficiency test. This time around the AFR didn't just spike to 16.7, it also dipped to 12.3 after going lean. Would make sense if the PCM is testing the cats and O2s.
There is a significant lag on the rear O2s. About 2 seconds (give or take a tenth). Not sure if this is normal or if it's signs that the rear O2s and/or the cats are no longer functioning as correctly as they should.
There also seems to be a significant difference between the drivers side vs the passenger. On this drive the PCM did the test twice, and both times there's about a full second delay between the drivers side dropping vs the passenger side.
Also, the rear O2s read a little high. Seems like at stoich they read around .7 volts. I could be wrong, but I would've expected them to be closer to .5?
Very curious to compare this to a healthy system. See how long the cat efficiency tests last and how quickly the rear O2s respond to the change.
Fairly certain this is the computer doing a cat efficiency test. This time around the AFR didn't just spike to 16.7, it also dipped to 12.3 after going lean. Would make sense if the PCM is testing the cats and O2s.
There is a significant lag on the rear O2s. About 2 seconds (give or take a tenth). Not sure if this is normal or if it's signs that the rear O2s and/or the cats are no longer functioning as correctly as they should.
There also seems to be a significant difference between the drivers side vs the passenger. On this drive the PCM did the test twice, and both times there's about a full second delay between the drivers side dropping vs the passenger side.
Also, the rear O2s read a little high. Seems like at stoich they read around .7 volts. I could be wrong, but I would've expected them to be closer to .5?
Very curious to compare this to a healthy system. See how long the cat efficiency tests last and how quickly the rear O2s respond to the change.
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Thread Starter
Joined: Jun 2014
Posts: 1,241
Likes: 47
From: Noblesville, IN
It is a stock tune, but I added it to the original post.
Thread Starter
Joined: Jun 2014
Posts: 1,241
Likes: 47
From: Noblesville, IN
Hoping maybe someone has more insight on this. If it really is just the cat efficiency test then I can tune it out for now and use it as an excuse later to get some headers and an off-road y pipe. Just want to make sure that that's it before I start messing with things/buying parts.