[MIKE91B]

This is normal. It's called DFCO. The FT go negative when in this mode. If my memory is correct the LT freeze relative to the FT Cell and the ST go to -25. This is all for fuel economy. The TCC remains locked during DFCO to keep the engine spinning. When you press the accelerator the injectors then fire again to continue combustion as normal. As for the noise I have no idea.

 

[MIKE91B]

Your thought process is correct! The variables are huge. First you need to know purge valve duty cycle or you need to unplug it to eliminate it as a variable. The fuel trims only used to determine fuel injector correction, this is not the only fuel source for the engine. The evap system is used to burn fuel vapor from the tank. That will drive fuel trims both high or low depending on the density of hydrocarbons in the vapor. I know the 2.4s and 3.6s like the back of my hand. If you need info on an equinox let me know.

 

[MIKE91B]

It's normal. 10% plus or minus is not even half of what the idle cells can handle. I'm not trying to scare anyone but you should see what some intake tubes will do to fueling. The measured airflow is the only input to WOT fueling, change the habitat of the MAF meter and fueling can change more than most would think. If you would like to temporarily disable the purge valve all you need to do is unplug it. I often do this when diagnosing lean or rich conditions. The purge valve creates a variable that will skew readings. My rule of thum is for every 2% of purge valve duty cycle you will decease the long term trims by by 1%. This is for idle calculations only as manifold pressure is lowest. Other variables exist but the major one is the EVAP system. If it was an issue you would have codes. P0171 and P0172.