1st. Do declare here the 2nd gen Fit is the wife's car which I very seldom drive excepting for servicing-test purposes.
A series of events occurred that began after my replacement of ATF with a non-recommended fluid . The wife took the car to another part of the state where temps at the time were near or over 100 degrees daily. Vehicle performed fine for several weeks. I will try to relate the bizarre events , and have eliminated all excepting an ECU that, after procedural steps are taken to "relearn idle", does reset to authorized RPM (idle) momentarily, but then reverts upward to 1200-1250 and "hunts" which suggests a vacuum leak.
Now the rest of the story.
Was summoned by spouse to attend to flagging Fit located in area of Texas where temps were extreme(at the time). Test driving indicated throttle erratic with little response and shift hard.
Multiple evacuations of ATF with replacement each time (with test drives to circulate) with improvement with each, totaling 3 gallon fresh Honda approved full syn ATF. By this time an engine miss had developed (#2) and throttle (for all purposes ineffective, unsafe and unable to achieve decent road speeds.)Transmission is now working fine with smooth shifts, (still) with engine miss.

Next, misfire cylinder was addressed and determined to be caused by spark firing (sideways) through plug boot directly into alum head (not through plug which had "some" but not excessive gap !) Records search revealed I had replaced plugs, COPS, and wires less than 2 years prior, and had less than 20K (est 10K) mileage. So, did replace all again. I suspect causation here of misfire was (over) use of anti-seize between boot and spark plug. However physical inspection revealed very small amount on boot.

The Throttle Body (TB) area , vacuum and coolant hose appeared fine with no visible defects. A new rebuilt TB was ordered and installed.
Inspection of the control device of the TB (black pvc housing) in which a multi geared "stepper type motor" rotates a butterfly shaft and the outer portion of shaft fits into the plastic housing and has no bushing or bearing for wear or adjustment . Over time (and heat in this case) pressure loading from the stepper motor wheel action, had worn the housing from round into an egg shaped opening. causing shaft to drop and bind. It is almost invisible, but enough to throw off the stepper motor.
TB was replaced with new unit, initially guesstimated at torque values and subsequent did acquire tool and set value (nominal).
The correct relearn procedure was finally found and followed which is a start up, no throttle or action on part of driver and after cooling fans have cycled (in my case 2 times) will ECU attempt to reset idle speed values. All Lights off and no loads on electrical systems conditions .
(I did leave daytime running lights on)
Above steps followed, ECU does go into relearn, settles down eventually to about 850 RPM, then kicks back up to about 1200 as the new learned idle speed.
EVAP line block into TB as stated for relearn has no effect either way
Upon test drive,
at low speeds throttle "hunts" and fluctuates between 1000-1500RPM. speed above 2K are fine, but bi-directional scanner indicates lean condition. These conditions suggest vacuum leak, but all examined and retightened.
Did use old silicone gasket between Intake manifold & TB.
Bi-directional scanner indicated EVAP value does function and fluctuates between vacuum and pressure depending upon action (flow of fuel) within tank at very small values.
Do surmise next step to be smoke test.
What am I missing here?
Thanks
TLeeBlanq