Hello, I have a 99 honda accord v6 EX. It had a bad idle air control valve a while back, I got that replaced. However I am still having intermitant idle issues. There is no check engine light, the upstream O2 was also just recently replaced by a shop. And an idle relearn done.
What I am experiencing:
1. After just starting the car warm it will idle really low i have not not looked at it with a scan tool but if i had to guess around 200rpm (heavy shaking/vibrating) if you let it sit it will eventually even out or if you give it a little throttle it sometimes helps.(again seems to only run rough at idle) though letting go of the throttle will still go low for a bit.
2. The RPM seems to dip most when idle when ever a load is applied, A/C turned on, put into reverse/drive. Again if you let the car sit for a bit it smoothens out and the dips are not as drastic.
3. The RPM when a load is applied once it has smoothened some, still dips to ~500 enough that there is a bit of a shake and the spec for this engine is 630-730. After a few seconds of the load being applied the rpm does creep up higher as if the engine is trying to compensate and does.(this happens each time a load is applied/removed consistantly no matter the running time of the engine)
4. The long term/short term fuel trim seems to very consistantly be -5 or more negative when idle. It will sometimes be higher( more +) but as the car idles longer it seems the more negative it goes.( not enough for a check engine light though)
Going down the road the short term will go possitive and negative as throttle is taken away/added to what i believe normal levels. This also contributes to the long term being less negative

I suspect a idle only misfire and maybe the cause is spark plugs because of number 4 it seems like maybe it is running rich because of unburned fuel due to lack of spark and once the fuel trim has adjusted/engine gets some momentum going it runs smoother.(not sure if that even makes sence)
Again it seems like its an idle only misfire however as far as I can tell from my basic elm327 + phone app on this year car misfire counts are not available and if there are misfires above idle it is not enough to feel the car seems to have lots of pep/smooth going down the road.

Edit: attached some logs and a image from OBD Auto Doctor. In the image im not sure what the unidentified test results are for.

 

So i think the problem is staring me in the face from the logs. At idle the Intake Manifold Absolute Pressure [kPa] is reading just outside/engine off value while running and stays that way in pretty much all of the logs. At idle it should be somewhere around 8. Engine off/outside Intake Manifold Absolute Pressure [kPa] is/should be around 28. Today I went out and confirmed that this did happen. It did drop to 8 unlike before but not sure its going to stay working. I now suspect a small leak maybe around the MAP sensor, maybe the oring that goes inbetween the TB and the MAP sensor. Being that it is 20 years old and cheap not a bad place to start.

 

So I have replaced the map sensor oring and it does seem to read accuratly now, however my problem still occures and still have a negative fuel trim at idle. Looking at o2 data I think there is something wrong with the downstream.

 

I know people say only upstream controls fuel but if my upstream is switching lean/rich why is my fuel trims constantly trying to pull fuel (negative) at idle if its going rich and lean

 

from the honda-accord-v6-1998-2001-sm-supp.pdf which can be found online(which i think is official honda?, "First Edition 9/2000 HONDA MOTOR CO. , Al l Rights Reserved LTD. Specifications appl y t o U.S.A. and Canada Service Publication Office A s sections with "include SR S components; special precautions are required whe n servicing. marke d section s are no t include d in thi s manual . 1998-0 1 Accor d V- 6 Supplemen")

"Primary Heated Oxygen sensor (Primary H02S) The primary H02S detects the oxygen content in the exhaust gas and sends signals to the PCM which varies the duration of fuel injection accordingly. To stabilize its output, the sensor has an internal heater. The primary H02S is installed in the exhaust manifold. By controlling the air fuel ratio with primary H02S and secondary H02S, the deterioration of the primary H02S can be evaluated by its feedback period. When the feedback period exceeds a certain value during stable driving conditions, the sensor is considered deteriorated and the PCM sets a DTC"

"Secondary Heated Oxygen sensor (Secondary H02S) The secondary H02S detects the oxygen content in the exhaust gas downstream of the Three Way Catalytic Converter (TWO and sends signals to the PCM which varies the duration of fuel injection accordingly. To stabilize its output, the sensor has an internal heater. The secondary H02S is installed in the TWC."

So it would seem that both sensors play a role in "the duration of fuel injection" which I would think has an affect on the fuel trim? And being that the two sensors vary greatly in reading that might explain the odd idleing behaviour and negative fuel trims possibly?