My 91 accord MT idling issue has been dragging on for about 2 years now. I've managed to live with it but recently its been consistently idling 500 rpms too high and at times surging between idle and 1400rpm.
I've read online that if the idle gets too high the ECU may precautionary cut fuel, so that explain the surging. So, now I have to find out why the car is consistently idling too high? Having said that, rarely the car will tend to idle much too low.
An interesting observation, is that the idle never surges while the car is coasting in neutral. That's why I believe the idling issue lies somewhere with the front most section of the intake system that sits before the air filter (see photo below). I can see a vacuum line runs up there and even a connector with wires. What's the purpose of this "device" that gets vacuum and power and sits pre-air filter? I've taken some of it apart and it seems to be some type of flap system that alters the air flow. I wonder if this "device" may cause idling issue if it malfunctions?

 

I take it you have pulled the FITV off and checked that it is clean and the screw is snugged down, per the FAQ at the top of the forum.
What have you done/replaced/repaired/checked while hunting down this surging?
FITV?
IACV?
Vacuum lines?
Gaskets?
Checked for air/vacuum leaks with your preferred liquid?
Verified correct operation of PCV valve and lines?
Verified the EGR valve or EGR ports in the intake runners are not leaking.
Verified no air/vacuum leaks around the injectors going into the intake manifold?
Verified upper plenum to lower runner is not leaking.
Verified intake is not leaking at head.
Verified TB to intake is not leaking
Veirfied TB throttle shaft rotates smoothly and is not sloppy.
Verify the various vacuum control solenoids are functioning correctly.
Verified various sensors (IAT, MAP, TPS, etc) are functioning normally.
Verified the engine harness ground has not become loose/corroded(located on thermostat housing).
Verifyed battery connections are clean and tight.
Verified battery is fully charged and no voltage issues.
Verified battery cables are in good order, and properly attached to the engine/fusebox/transmission/body.
Verified engine harness, connectors, and pins are all properly attached and not loose or corroded.

The device in front of the air cleaner housing is the air intake resonator chamber. This is to quell drone and other air intake noises. This is not open to outside air directly, it acts as an intake muffler but more like a helmholtz resonator to prevent standing waves from backfeeding and multiplying in sound. There is a smaller chamber on he air intake tube itself between air cleaner and TB. J shaped.
Not only do they silence air intake noise, they actually aid in improving the engines volumetric efficiency, improving overall power.
What the kiddos like to hear with their raspy CAI/SRI intakes is actually the sound of inefficiency.
You can easily bypass this system if you want to test it.
Remove the air cleaner housing.
Remove the fender lineing.
Unbolt the large chamber and associated tubing and remove.
PDisconnect the vacuum tube feeding the air intake control valve and plug it.
Reinstall fender liner and go for a drive.
What you will notice is your air intake is louder, like the kiddos, but I highly doubt you will have anything affecting your idle, unless the vacuum hose or control valve diaphragm was leaking.