Just got a 2000 Accord SE off a neighbor for free with 83k miles. Did an oil change first thing, and checking the car over before I take it out. It starts instantly, warms up and drops to idle as it's supposed to.

However, if I step on the gas a little, the RPM's drop to below 500, and if I let off the gas, it slowly gets back to idle RPM. If I gently feather the throttle, I can get it to come up over 1,000 RPM. If I let go, it dips back below 500 and then comes back up to normal idle.

Any ideas?

This problem also happened before I changed the upstream AF sensor. It was changed because I got a code 1166, which is now gone.

I haven't changed anything except the AF sensor

 

The catalytic converter might be clogged up. Try removing the oxygen sensor to let it breathe and see if it runs better.

 

So, I removed the upstream o2 sensor (AF sensor), and there was no change in behavior. After warming up, at idle, pressing the gas made rpms dip to almost stalling. So, the cat is probably not clogged. There are no bogs above 1000 rpm, only when you let off the gas it dips back down to almost stalling, and slowly comes back up to idle. Putting the car in gear or neutral would also dip to almost stalling before coming up to a very low idle.

Next, I decided to take off the intake duct, felt around with my fingers inside the throttle body and found the walls to be covered in black residue. I sprayed a little cleaner in with the butterfly open and wiped everything.

This seemed to may a difference. After warming up, at idle, pressing the gas made a dip, but much less than before, and it was no where close to stalling. Also, putting the car in gear or neutral did not come close to stalling. And it runs at a slightly higher idle than before

Should I remove the throttle body for further cleaning? I'm nervous to do so, to prevent breaking old hoses, clips, etc. Is it time to change the idle air control valve, or maybe the passage is restricted from EGR and PCV fumes?

 

What is the history behind the car?
Was it a freebie because of the running condition? Or has it just been sitting for a long time?

Check for any stored codes. Don't 'clear' any warning lights by unplugging the battery. Those codes will help you diagnose the problem.
Pull the air filter out, if it doesn't pass the light test replace it.
Pull the spark plug wires, is there any oil on the wires/spark plug tube?
Pull the spark plugs, verify the part number on the plug is correct for your engine. Look at their color/condition as is from the engine. Do not try to clean them, and then compare them to a color chart such as https://www.ngksparkplugs.com/about-...d-a-spark-plug if the plug looks normal, use this chart to help see where in the 'normal' spectrum the plug is. http://www.miniplane-usa.com/images/sparkplugcolors.jpg
Plugs will tell you the condition quickly without any specialty tools. To prevent damage to the threads in the head, start the engine and let it warm up a bit, you don't want the engine fully warmed up, and you don't want it cold. If the valve cover is warm but cool enough to touch that would be a good temp to pull the plugs.
If there is a lot of oil in the air intake tract, there is probably a blow by or crankcase over pressure issue, causing oil mist to be blown out of the fresh air intake hose between the valve cover and intake tube.
Pull out the PCV valve and verify it rattles, if it is overly oily clean it, the hose, and the port on the intake out with throttle body cleaner.
Verify the oil level is not overfull. Car on level surface, engine off, pull out dipstick wipe it down reinsert. Pull it out and check 3X to verify oil level is correct.
Verify no damage to the exhaust of anything plugging the tail pipe.
Next you will want a volt meter.
Locate the MAP sensor. Back probe, you will want to locate the sense wire. There will be a 5V reference(constant input), ground, and the sense(varying output) wire. Sense wire is what you want to watch the voltage. With the ignition turned to II(ON) the voltage should be ~3V. With the engine running at idle the voltage output should drop closer to .5V, opening the throttle the voltage should gradually increase with decreased vacuum in the intake manifold.
If the voltage fluctuates erratically(not with the engine running condition) or does not change voltage at all the MAP sensor or wiring may be faulty.
If the voltage fluctuates with the engine behavior it may be there is an internal engine running issue.
Compression check would be ideal to verify the condition of the engine. An abnormally low or high reading would indicate a mechanical problem.

 

You may want to run a compression test to make sure the engine is in good condition. If the spark plugs haven't been replaced in a while it might be best to get some new Denso.

 

Just for giggles, check the color of the wires going to the map sensor....lets remember that the map and tps sensors have the same connections and get swapped quite often and will produce an issue very similar to the one the OP is describing.