We are at about 500 m here (1640 ft), at hot idle I see about 30 kPa.
With AutoTap I can't see if the fuel learn is on, or at least I don't know how (do you know if Autotap can do that?)

I adjusted the values to fit the LS1Edit IFR table. It's easier to compare / change the values. According to the scanner the pressure is in kPa (??) and not in inHg

Ah, that says a lot. If your going to trust your tune changes to what your scanner tells you can be a mistake.
I quit using Atap 5 years ago when I saw too many things wrong. Fuel learn is a EPA PID since they dictate when trim values are averaged in or are ignored and you need to know what PCM has either commanded injectors off or went to min pulse width.

It is an important PID to know when engine is going into decel or is in decel or when PCM is truely into power enrichment mode

Ease Enhanced OBD-II Powertrain Scanner ?
Are there other producs you would suggest?

I see what you mean by "you need to know what PCM has either commanded injectors off or went to min pulse width". Right now I see LTFT / STFT values that don't make too much sense.
After a while I'm at idle the fuel trims go negative without an apparent reason (?)
I have #36 injectors and at minimum pulse width they could still put too much fuel in the engine.

Also the averaged cells change depending on the wheater. It was 0% at 60F and +2/+3% at 40F.
I wonder if this is the result of a wrong tune or of wrong data from the scanner (???)

What a mess!

I suggest the Ease scanner because they are a tier 2 suppler to GM so their scanner was designed with GM engineering input and information that is not known by the public for their enhanced PIDs plus how the VPW functions.

If interested you can purchase it via our frontstore and this is the model you would want :

Ease GM Enhanced OBD-II

Your autotap does less then about 10 frames a second where with Ease you get 40 F/ps.

In your case PCM cycles are then missed where with the Ease your getting far more data cycles.
Also the Ease supports all GM PIDs, has bi-directional, full EPA IM snog tests and O2 tests.

OK, let me think about that.

Here another question: is there a natural reason of having different LTFT's depending on the wheater? Temperature? Air umidity? Pressure?
Or can it be related to a tuning problem?

The difference between 30F or 70F is 4 - 5%
Is that normal? Should I just take something in between?

Sure weather will have a effect.
Try using the weather calculator on my website,
direct is Weather cost to performance

Make weather changes with it and see the effect it has. Hot air adds to the correction factor of HP/Tq which is a loss of performance where colder airmass would increase performance so it clearly has effect to AFR and fuel trim values

You see a hugh effect to weather to AIR effect on performance.

Clearly you know in summer heat engine does not perform as well as nice fall or spring weather so air quality effects AFR and reason why using the MAF is much better adjusting tool for the PCM then those people deleting the MAF and use VE speed density as to how PCM determines how much fuel to add to qualty of airmass.

As to tune corrections I slant on the lean side as long as there is no knock pulling timing so there is no rule that LTFTs must be a perfect zero being as long as STFTs are +/- 5% PCM ignores LTFTs and does no correction to them

Interesting calculation! As I expected Dewpoint has a big influence... This is the only real problem of a MAF: it measures mass. Air mass but also humidity mass.

So with umid air the MAF will "see" more air than what it really is, causing more fuel to be injected. At this point the O2's will notice the difference and cause negative fuel trims.
Right?

In my company we have a couple of gas flometers: they are similar to a MAF in their function.
There's no chances to have a correct reading if we don't have a drier installed before them!

So what to do? Install a big drier in front of the car?

I do not see that as a problem for in fact that being MAF can sense humid air and adjust for it assures the PCM is properly adjusting for the quality of air.

PCM is not a perfect realtime monitor and adjuster since it only has to make changes over time and as long as PCM keeps AFR to within 1.5 times the FTP ( Federal Test Procedure) rate then all is well.

Really having the fuel trims ramp up/down (rich/lean) is good for that assures the O2s are properly also ramping above and below .450 mVolts.
If fuel trims stayed flat then O2s also would and that would confuse the PCM thus as long as STFTs are within that +/- 5% then engine is running perfect and forcing flatline of fuel trims would not make any more performance.

Take a simple driving up and down a hill, if PCM was realtime the trims would be lean, then perfect 0 and then rich going down the hill. If PCM realtimed those changes the engine would not run smooth that driver could feel and why PCM only reacts to LTFTs once every 10 minutes and once a second looking at O2 values.

Is it better to consider LTFT and STFT together?

correction = (LTFT B1 + LTFT B2)/2 + (STFT B1 + STFT B2)/2

This assuming that x% correction for STFT has the same weight for LTFT also.

Example: +4% LTFT and -7% STFT means a total adjustment of -3% (for a particular FTC)

OK, I'll answer myself: see here the result of 2 very long scans (3 hours each, visiting my parents for Easter).

Notice the IFR curves calculated from the LTFT's: they are different.

The other 2 IFR curves are the result of considering LTFT's + STFT's and are the same!
This means (to me) that the formula above (averaged sum of LT and ST) is correct.
As stated by JR LTFT will change only if STFT are higher/lower than +5/-5%. Considering LTFT alone can lead to wrong values (as I personally experienced!).

The 20-30 kPa area is pretty strange

I also wonder if and how much the EGR influences the whole story

Stefano