TBSS MAF Over Reporting Airflow

[LastCall]

I did a MAF only tune on an E67-based LS2 a few weeks back, and all was good. Used a wideband and got the AFRs within a few percent of commanded. It ran pretty good. The ECM was recently removed to install in another TBSS, and then back into the original. That triggered a crank sensor relearn DTC. Shortly after, two codes came up. P0172 and P0175, basically both banks rich. I logged it and noticed the MAF was reporting 23-34 grams/sec at idle (850 rpm). I looked at the tune and it was at the 4400hz mark, which did indeed reflect 24 grams of air. Going through the logs a few weeks back, it was idling around 3300hz and 11 grams/sec. So I assume the long term negative fuel trims (pegged at -25%) are from the excessive and incorrect airflow being reported to the ECM, and the O2's trimming it to the max. I reset the fuel trims in the scanner, and they were still -25%. I'll post a before and after log as well as the tune.

So the question is: What would cause this? I would think bad MAF, dirty MAF, or MAF wiring? I don't see how the ECM removal and reinstall could do it.

Thoughts anyone?

[joecar]

Some $0.02 thoughts:

When it was previously tuned, were the LTFT's cleared...?

When it was being borrowed, was it edited/flashed...?

Try cleaning MAF with CRC MAF cleaner... MAF could be bad and still appear to be working. MAF wiring problems will cause DTC's.

Please do post logs.

[LastCall]

Good news, there was a restriction (electrical tape-lol) on the maf screen. Yes I cleared LTFT when I logged it, but ran it in open loop with wideband when I tuned the maf curve. Swapping the ecm's required a crank relearn, so that. Will report back when I hook back up and log.

Thanks for quick response JoeCar!!

[joecar]

good job lol, electrical tape, where did it originate from...?

( this shows how important visual eyeball inspection can be )

[LastCall]

The maf was to small for the intake plumbing, so there was electrical tape on the mounting surface to make it fit snug.

Yes it does highlight the importance of a visual inspection.

[LastCall]

So raising this one back up - same truck.

So I updated the injector data from stock to Fast 36#. Since the original MAF curve was calibrated with 36% injectors, but stock data, it had to be reworked. I did that and got some very interesting results. To get LTFT part throttle fuel trims within a few percent, I had to add about 40% to the MAF curve. These injectors were 30% larger, but not sure where the other 10% comes from. Difference characteristics at idle? Weird.

One issue I see now is that idle ariflow as measured by the MAF is 15-16 g/sec at 850 rpm. The LS2 is heavily cammed, but that seems like way too much idle air.

What should I check?

[joecar]

Are the FAST injectors 36 lb/hr at 3-bar...?

Are the other injectors 36 lb/hr at 4-bar...?

[LastCall]

Yes, 36 at 3 bar. I converted the values with a spreadsheet based on 4 bar pressure. I'll post the tune later.

One note, I could not find injector voltage offsets, only in gen 3 format. And I read you couldn't copy, paste, and interpolate between 3 and 4 voltages because there like values don't correspond to one another. Redhardsupra posted about it. That still valid?

[joecar]

...

One note, I could not find injector voltage offsets, only in gen 3 format. And I read you couldn't copy, paste, and interpolate between 3 and 4 voltages because there like values don't correspond to one another. Redhardsupra posted about it. That still valid?

I don't remember... do you have a link to RHS's post...?

[LastCall]

Yes, see post 34.

Correct IFR tables for LS3/LS7 injectors in Gen III
https://forum.efilive.com/showthread.php?t=11498