Vehicle Details:
1999 Honda Accord EX Sedan
2.3L VTEC ULEV AT

Problem Description/History:
Vehicle sat several months with a dead battery. Upon replacement of battery and positive cable, an idle surge appeared at startup and continued after vehicle warmed.

I have since resolved the idle surging at startup by replacing the IACV. Also cleaned throttle body. However, the vehicle still idles a bit high (1250 RPM), and continues to surge (up to 1550RPM or so) after warming up for about 5 minutes. Idle surge only appears when vehicle is in neutral or park, and disappears when in any drive gear (R,D,3,2,1)

Coolant system has also been drained, refilled, and repeatedly bled. I have replaced/isolated most of the vacuum lines connected to the intake plenum, to no avail. I have thus far been unable to locate a significant vacuum leak, but it's still a possibility.

The strange thing about this problem is that if I wait until the idle begins to surge, then place an additional electrical load onto the system (like headlights or cooling fans), the idle surge disappears within seconds, and will subsequently reappear within seconds once the electrical load is removed from the system again. There is definitely some sort of correlation between the surging idle and electrical load levels.

Battery is new and appears to be charging normally. Checked for voltage levels with multimeter across positive and negative battery cables with car off (12.9v steady), idling without surge (14.2v steady), and idling during surge (14.2v steady). I have not yet performed any further testing of the alternator or other electrical systems.

No engine codes appear to be generated by the idle surge.

I have not yet taken the car on the road, but will test that soon. I assume it will be okay since the idle surge only appears when car is in Park or Neutral.

This issue is a strange one. Any ideas, thoughts or suggestions are certainly welcome.