Computer Diagnostics and Tuning Technical discussion on diagnostics and programming of the F-body computers

LT1 randomly decides to go lean

Old 05-17-2019, 03:32 PM
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LT1 randomly decides to go lean

Ever since I got my wideband, I noticed that after let's say 10 minutes of driving, my car suddenly decides it wants to run much leaner and I can definitely feel a huge loss in power and overall performance. It may have done this before but I may have thought it was just in my head.
This wideband confirms it, and i've uploaded a log here to show it. At around frame 890, the car just decides to randomly go lean. I will mention that not everytime I drive it does it do this, but a good majority of the time. If I turn off the car for a few minutes and start it back up it seems to be normal again, but then after 10 minutes of driving again it does this again. Anyone have a clue?

Uploaded log in csv with all the major pids. Any help would be appreciated.
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File Type: xlsx
going lean 3.xlsx (173.3 KB, 7 views)
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Old 05-18-2019, 11:43 AM
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Re: LT1 randomly decides to go lean

Downloaded the file. I'll try and take a look at it in the next day or so.
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Old 05-18-2019, 12:12 PM
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Re: LT1 randomly decides to go lean

Appreciate it.

Also here's an update. I uploaded a tune forcing open loop all the time and it still randomly went lean about 15 minutes in.
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Old 05-20-2019, 03:03 PM
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Re: LT1 randomly decides to go lean

Generally seems to happen when you open the throttle. Look at frames 995-1024. You open the throttle, the wide band leans out, close the throttle a bit, wideband shows some recovery, then you open the throttle more and it goes as lean as 17.0:1, and averages 16.3:1. Yet if I average the narrow band sensors over the same range of cells, left side averages 659 mV and right side 825 mV. That isn't lean like the wide band.

Not sure what in the program adds a bit of extra fuel when it senses the throttle opening. But it should.... like the accel pump on a carb.

The relationship between readings is not easy to establish, because there is no time stamp on the log. I have no idea if its reading 5 frames per second or 1.2 frame per second. The data points in each frame are not taken all at the same time. The PCM is stepping through each item one at a time, so if the scan rate is slow, the data disconnects.
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Old 05-20-2019, 03:23 PM
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Re: LT1 randomly decides to go lean

Let me re-upload that log with timestamps for ya. Thanks for helpin a brother out.
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Old 05-20-2019, 11:49 PM
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Re: LT1 randomly decides to go lean

Here you go. Hope that helps.
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going lean.xlsx (191.8 KB, 5 views)
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Old 05-21-2019, 10:31 AM
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Re: LT1 randomly decides to go lean

This appears to be the same file you linked above, just with the time stamp column(s) added. Is that correct? I would prefer not to start over with a new file, because I've already reformatted the first file in a more useful configuration, and marked up the suspicious areas for further checking.
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Old 05-21-2019, 10:33 AM
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Re: LT1 randomly decides to go lean

Same file sir. Just added the timestamp!
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Old 05-23-2019, 02:56 AM
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Re: LT1 randomly decides to go lean

I'm wondering if the wideband sensor is just bad.

About a week after it started doing this is just reads full lean (3 dashes) all the time. Stuck at 18.4 in logs.
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Old 05-23-2019, 08:55 AM
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Re: LT1 randomly decides to go lean

That's why I posted the average (not lean) readings for the stockers vs. the average (lean) readings for the wide band. And that's where the slow data recording becomes an issue. We are seeing 1 frame of data every 0.7 seconds. We can't see what happens in between, and the PCM is updating many times in between the readings we see..

Another issue may be lag time between stock and wide band readings. I'm assuming stockers are in the header collectors. Where is the wide band?

I didn’t have a chance to look at the data log yesterday. Crappy weather here in the north east today. I'll have time to look. There does seem to be a trend where the wide band goes lean when you press the throttle. How does it feel when you try to accelerate? Does it feel “flat” on tip in? That could be a programming problem.

Let us know what you find out on the health of the wide band.
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Old 05-28-2019, 04:19 PM
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Re: LT1 randomly decides to go lean

So I got a replacement wideband, and right away it started working correctly. I did notice again, after about 20 minutes of driving, it would lean out to dashes, but if i'd get on it real hard or blip the throttle, it would go back into range. But just light accleration or stopping it would stay at full lean.

Then after about 10 more minutes, it decided to start going back in range again. I'm almost wondering if there's something messed up in my tune that's doing this or potentially leaning out so bad after awhile that it's destroying the o2 sensor. I only got about 100 miles out of the first one....

By the way, Solomon did the tuning.
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Old 05-30-2019, 11:45 AM
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Re: LT1 randomly decides to go lean

How old are the narrow band O2 sensors? Could they be going bad and feeding the PCM bad information?
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Old 05-30-2019, 04:47 PM
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Re: LT1 randomly decides to go lean

You didn’t answer how it feels when you try to accelerate gradually? Does it feel “flat” (hesitate, lag, etc) when the wide band goes lean? ..... compared to how it feels when it doesn’t go lean? Because as I indicated, I believe that may be a programming issue.
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