Back in December 2014 while stopping at a red light I noticed that rpm dropped below typical "idle in drive" RPMs (I did not pay much of the attention to this at this time since it happened when engine was warming up
About a year later (2015) the issue came back and stayed entire drive cycle (it was a second start up for the day). The car had tip-in hesitation and significant "jolt" when shifting to drive from park; even in park the RPMs were lower than typically are. There were no misfire, however. Overall the car was acting like an old car with vacuum leak or lean mixture.
Next, in 2016 I replaced coils (Hitachi), however the problem still was showing up at random times (like once every 3 mounts or so.
Finally last week, it showed up again similar but not as severe as in 2015; although it persisted for few miles of drive, it was random in nature: absent on one stop sign and present on other, 50 feet away.
So I plugged in my scanner and managed to capture this issue yesterday ( when at red light) , it was very brief and happened once around 84 degrees engine temperature.
I noticed that when the RPMs were going down (from 700 to 630) the STFT was -11 then immediately switched to +10 and stayed this way for 5 seconds, then it rebound back to 2.3 (and idle went back to normal) Again there was no jerking (misfire).
I was not able to capture MAP or ECT reading at this time
Assuming they were normal, I will guess so far:
Injector had stuck open for a moment
Gasoline saturated Charcoal canister and/or issue with purge valve
Air-fuel ratio sensor momentary fault (but it typically will not show negative fuel trim, only positive correction leading to P0171 code)
PCM bug (or solder joint crack)
Will scan more and report back
At this moment I don't think its idle control related, because, in theory, the fuel mixture should not be affected by throttle plate position

 

Theory, huh?



I've had several cars that developed similar symptoms if the throttle body/butterfly was dirty.

When was the last time you cleaned yours? Costs nothing to do.

Clean it & report back.

 

That is the same thought I had reading your posting. I have had several cars do the same thing and cleaning the throttle body fixed the issue.

 

Honestly it feels like that.
Last time inspected 15,000 miles ago
If EVAP purge valve and EGR check out I'll clean it out

 

Just to rule out "over traveling throttle plate", I'll monitor the TPS signal to make sure it's the same during normal operation or during "low idle".
Also will check MAP reading to see if it differs from typical.
For comparison with disconnected coil the car shakes and idle tries to compensate for dead cylinder.

 

Update: inspected throttle bore and found a ring of dust and dirt around the plate (no, it was not molybdenum!)
Cleaned it out as well as a layer of fine dust inside of the airbox (which came from the grommet around the IAT sensor); changed the grommet too.
Also inspected the PCV valve (by disconnecting it from the intake fitting and applying vacuum/pressure to it using bulb syringe) seems to be working fine
Will test and report
PS I have not done idle re-learn procedure, but will do it next if low idle/cutout persist.
At least I'm happy that this issue is not transmission (TC lock up stays on) or ignition related