Help me diagnose an unexplained AFR change

cidsamuth

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2008 Mustang GT with Kenne Bell Stage 1, Kooks Longtubes, high flow cats.

In Nov 2017, I had it dyno tuned. The tuner set air/fuel at 11.7:1 at WOT. A few months later, I installed an Innovate wideband sensor and ran it to an Aeroforce gauge. It confirmed 11:7:1 at WOT.

So, I set the Aeroforce in scan and didn’t pay much attention to AFR this last year. In the last week, I started paying attention to it again, and it gets down to 11.1:1 at WOT.

The changes in the last year since the dyno:

1. Pulled the Autolite plugs put in by the previous owner and installed Brisk. The old plugs were at .045 gap. At the tuner’s recommendation, I did the Brisk at .035.

2. Installed MSD plug coils.

3. T56 Magnum with RST clutch.

4. Went from 3.55 to 4.10 gears.

5. Upgraded the HE and intercooler pump.

All other changes are either cosmetic or related to the brakes and tires.

What I’ve done to diagnose:

1. I recalibrated the wideband sensor. No change.

2. I put the OEM coils back on. No change.

3. I pulled the Brisk plugs, cleaned, inspected, and reinstalled. No change. They looked great with no evidence of running rich. The tailpipes are not sooty.

The car feels like it always has. I considered the gauge might just have gone faulty, except that it hovers around that magic 14.7:1 ratio when cruising or idling.

I’m starting to wonder if I’m going to have to pay $500 again to put it on the dyno again.
 

cidsamuth

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Not sure why the font is so hard to read on my post.

2008 Mustang GT with Stage 1 Kenne Bell, Kooks Headers, and high flow cats.

In November 2017, it was dyno tuned. Air/fuel was set at 11.7:1 at WOT. A few months later, I added an Innovate wideband ran to an Aeroforce gauge. It confirmed the 11.7 at WOT.

So, I didn’t pay attention for the last year. When I started looking again, I see it is now 11.1:1. This is consistent.

The only changes in the last year:

1. Changed plugs from Autolite gapped at .045 to Brisk at .035.

2. Added MSD plug coils.

3. T56 Magnum trans

4. Went from 3.55 to 4.10 gears.

5. Upgrades the HE and pump.

To diagnose, I took these actions:

1. Calibrated the wideband. No change to AFR.

2. Reinstalled the OEM coils. No change.

3. I pulled all eight plugs and confirmed the .035 gap.

All plugs looked great with no indications of a rich condition. The tailpipes are not sooty. Car continues to run great. If not for the change on the gauge, I wouldn’t even be thinking about this.

I guess the gauge could be off. I could adjust WOT AFR on the SCT handheld, but I’m hesitant in case it’s the just gauge inaccurate.

I’m afraid I will have to pay $500 for another tune to know for sure.
 

decipha

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tunes dont just go bad or change, best thing to do first is to verify your fuel trims arent wayy off to know its running correctly. But perhaps your fuel pump was weak before it could very well be slightly richer now.

in any case u dont have to pay for another tune to find out. Simply pay the minimal dyno pull fee ~$100 for 4 pulls? Or just buy another aem digital wideband and try it out ~$160

If u do need to go back to your tuner they should charge u a mininal fee for that very slight revision if anything at all.
 

01blckcobra

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Datalog it to see what is happening... Also Innovates are touchie. Try re-calibrating?

Usually what I've found with innovates they start to slowly read rich then the sensor dies out of nowhere. Also will have longer warm-ups.
 

SecondhandSnake

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Yeah I second the sensor. Just because you recalibrate it doesn't mean the sensor itself is accurate. Is it a Bosch 4.2 or 4.9 sensor? I've been using the 4.2 and it's not the most robust. Lots of high temperature/pressure and rich operation seems to degrade the sensor fairly quickly.
 

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