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Hello everyone! I have been dealing with this issue for months now and have learned more about this car than I ever thought I would. Read many forum posts between some of the major Civic and Honda forums and I just can't get this issue solved and I am just about fed up with it so I am going to break down and ask you all...
The explanation might be a little long as the problem has changed twice since it began.

The car:
1997 Honda Civic EX Coupe, engine D16Y8, 5 speed manual. Mods are unknown. I haven't modded it myself and there are no obvious mods from the previous owner. I have had the car for almost 6 years and it's given me very little trouble other than a failed alternator (replaced myself about 2 years ago), and now this.

The problem:
Originally, the car had a fairly normal idle, around 700 rpm. Occasionally this would drop to less than 100, could feel every piston blow and every time I thought for sure it would stall. Alternatively, it would occasionally jump up to around 2000 rpm. Each time the car would start normal and some time during driving it would drop or surge (one or the other) and stay there usually for the rest of the drive. It didn't bother me too much other than the fact that I thought at some point this problem will get worse and cause it to stall in the middle of driving (with my luck would be in the middle of a long drive).

The second part of the problem started with a horrible (but unrelated) oil leak from the Vtec solenoid. It suddenly started puking almost the entire sump out in the span of a week. The upside to such a bad oil leak was that it forced me to really work on the car and the oil leak was easy to find. A friend and I worked on replacing the gasket on the solenoid and diagnosed the idle issue at the same time to be the IACV. So we swapped the IACV and the car ran so smooth... for a day. The next day on the way to work (I was almost there thankfully) the car choked and felt like something yanked the car backwards. The check engine light came on nice and bright and the car limped very badly the rest of the way to work. Starting from a stop it would start to choke and feel like it was getting no gas, and then would jolt and jump hard. After work, it didn't do that at all, drove home and went to the closest autozone or Oreilly or whichever and had them check the CEL codes. It listed the MAP sensor, IACV, Manifold absolute pressure, and something about vacuum or pressure leak in the fuel system. After resetting the codes, the last two never came back again at all. The first time came back once or twice after clearing the codes but not changing anything else yet.

The third part of the problem is after clearing the previous codes, checking all those connections and testing the individual sensors, leaving the battery disconnected over night hoping to... I don't know, reset the computer or something. The CEL never came back on again but it was idling really high, usually around 2500 or even as high as 3000 rpm. If I start driving the car it drives more or less just fine other than issues about the high idle, at some point during driving I come to a stop and it starts surging between normal idle and about 1500. I tested this in the drive way, I let the car run until it warmed up and the thermostat opened, it runs very smoothly but at 3000 rpm consistently until I touch the gas pedal and only after hitting the gas up above where it idles, then it suddenly drops down and starts to surge again, I let the car cool down, start it, it runs smoothly around 3000 rpm, I let it warm up, I hit the gas, and it starts surging. And this is currently where I am at with this.

I have tested the MAP sensor and it seems to be producing proper voltage (I replaced it with a few different once that I found at a pick n pull just to make sure and between the 4 total that I tried, nothing changed).
I tested the new IACV and it is producing proper voltage.
I checked for vacuum leaks
I checked coolant levels
I checked for air bubbles in the coolant (as per Eric the car guy video about honda idle issues)
I tested the TPS and it is producing proper voltage.
I tested the primary oxygen sensor and it is producing proper voltage.
I checked FITV (nah this engine doesn't have it as I found out after a poke around where it should be and a quick google search)
I broke down and bought a monitoring tool that can monitor live data and at least as far I can understand everything appears to be producing the proper signal voltage according to the Chilton manual/google.

Have I missed anything or might I be facing a failed or failing ECM? I would think the issues would be worse or the car would not be running at all if this were the issue...
Also understand that I am not a mechanic and not a car guy, I am a computer guy, I write code, I sit at a desk all day, I just like solving the problems myself and the mechanics quote me at $150 to $200 an hour for diagnostic and this peeling red civic isn't worth that much to me.

Sorry for the long post, I appreciate any help and input. I don't want to sell the car in this condition and I really don't want to take it to the junk yard.
Thanks in advance!

 
Rep Power: 366

Check battery/alternator?
electronics go crazy with varying voltage/power and light a Christmas tree on the dashboard.

This issue is more common in the 7th gen cars, but worth a try.

 
Rep Power: 0

Well, both the battery and alternator have been replaced since I got this car and when they were bad, they didn't cause any problems like this. Also there are currently no indicators lit up on the dash board at all. It seems to think there are no problems or issues. No check engine light, no battery problem indicator light, but the lights do work, the seat belt light lights up when I am not buckled, the emergency brake light, headlights and headlight indicators work fine. But no CEL or any other problem indicator.

In any case, I will check the voltage outputs of both of those and I will get back about it later today.
Thank you

 
Rep Power: 242

DId you use an OEM IACV?

What do the plugs look like?

 
Rep Power: 0

I doubt it was OEM. I picked it up at O'reilly's or Autozone I think. The plug is round, not oval shaped one. Or did you mean if the plugs look damaged? They all appear to be in fine condition, no visible corrosion. Couldn't find any obvious breaks or wear spots in cables either.

I did keep the old IACV. Do you think I should put it back in? And if so, what is the proper procedure to clean this thing considering it is one of the ones that is encased and can't be disassembled. Cleaning it previously made no difference, but that was before this high idle and surging problem.

 
Rep Power: 242

Aftermarket parts have shoty reliability and have been know to cause problems when new.

What do the spark plugs look like?

I have never cleaned one but I am sure there are videos online that can show you how to properly clean it. Might be worth a try to clean it and swap it back in to see what happens. Cheaper then buying a new OEM one for now.

 
Rep Power: 0

Update: I very thoroughly cleaned the old IACV with brake cleaner/simple green as per some online tutorial I found. I cleaned it about 4 times as black gunk continued to come out of it. After allowing it to dry, I reinstalled it into the car and reset the ECM and performed an idle learn procedure that I found on another forum post (that is, letting the car warm up to operating temp, pulling the fuse and letting it sit for a few minutes, and turning the car on with no accessories running for 10 minutes.)

The results of this change is that the surge is gone and so is the constant high idle. So yes it seems the new IACV must be faulty. However now I am back at the beginning of my problem. On the way to work this morning the car was idling fine at around 700. After work when I started the car, it began at around 700 but about a few minutes into a drive I came to a stop light and it began idling at 2000 to 2500 rpm. No surge, but still the problem that the idle does not consistently remain at a proper level. I stopped for gas on the way home and after starting the car up again it immediately dropped to around 100rpm and remained really low at every stop until I got home.

Is it worth trying to get another IACV? Those things are pretty expensive to just throw at the car... Should I try finding an OEM one at pick n pull or something? Or is there another component I should look deeper into involving this original issue?

 
Rep Power: 221

There's little ports in the main throttle body mouth, if you cover one, or both? (Sorry it's been a while) see if the idle drops to normal. If it does the IAC motor is bad/stuck on a chunk of something. Junkyard will be cheapest however you'll need to go through the cleaning process again.

 
Rep Power: 0

I tried that once before and it had not made a difference with the old IACV (I didn't try it with the new one which was really screwing with the car I guess). Also, if I recall correctly, this engine has only the single port in the throttle body mouth. I think the reason it didn't make a difference though is because when the car first starts (with the old IACV) it idles normally. So I suppose what I would have to do is drive until the idle is off and drive home and park and leave the car running to test it because stopping the car and turning it back on seems to temporarily reset it.

I will try to make a junk yard run this weekend and see if I can come up with an OEM valve.

 
Rep Power: 242

Great job so far with what you have done. I agree with Colin see if you can play with stuff while the car is acting up. A low mileage junk yard one would be better then a aftermarket one.

 
Rep Power: 0

I wish I had someone to tell me that before I got an aftermarket one. That made the whole problem a lot worse.

 
Rep Power: 221

We tell people that in this forum probably at least twice a day, keep us updated, hopefully you can get it sorted out