Hello. I have a 2001 ES5 with D16W7 and when the engine is cold, I have quite good torque at low RPM range. However, after it reaches operating temperature, I feel it becomes slightly weaker under 2500-3000 RPM with partial load (light acceleration) and sometimes my idle also drops for a few moments (not suddenly, but smoothly, like the PCM mistakenly starts to lower the idle below normal).
Neither happens immediately after the PCM falls back to normal idle, but only some time later when everything has completely warmed up. If I accelerate hard, I have no issues, the engine pulls very smoothly and extremely powerfully (sometimes it can even spin my tyres after starting normally, but then getting around 6000rpm in first gear on wet road, and it also has a lot of power beyond highway speed too) .
Two more things:
- As the engine warms up, it gradually starts to give some deep periodic intake sound proportional to the engine speed independent of load, which smooths out around 3000-3500 RPM (VTEC switchover?). The sound is nice and has a healthy impression, so it may just be caused by the air resonator chamber or the VTEC-E 12 valve mode (or it's low load lean-burn submode), but I have no comparison base (I don't know anyone in person who owns any Honda with a D series engine). And because the sound also comes when the engine is warmer in the same RPM range, and couldn't be heard when the engine is still cold, I can't rule out the suspicion that there might be a connection with the issue.
- Sometimes the engine tends to lightly knock for a few seconds (just the high frequency sound of colliding flame fronts, no real hard piston kicking knocks) when suddenly encountering higher load at lower RPM's (1500-2500) with really warm engine (so the same conditions as for the base issue, except that I feel the torque loss after warming up with higher octane too)

I wouldn't think of compression loss because of the knocking and because it mainly occurs with 95 octane fuel and feels it's occuring equally in all cylinders, but much rarely with higher octane fuels which have better compression tolerance.

My long term fuel trim was always negative when I checked it some time ago, around -2-3 percent's. I have no CEL codes.

I read that old, worn-out O2 sensors could cause issues like that.
What do you think, am I on the right track?
Does the PCM always start with closed-loop, then switches to open-loop only after the engine reaches maximum operating temperature?
Does it fall back to closed-loop when I accelerate with wide-open throttle?
Does open-loop mode affects idle running?