Checked the idle and the idle gets higher with the screw more screwed out.

 

My current theory is that some sensor or something is causing the ECU to signal the IACV to make that surge, since the car idles perfectly when the IACV is unplugged. It's a new IACV...

I am tempted to take the intake manifold off and throttle body off to inspect gaskets and function and check for cracks etc but I don't think any of these things are an issue due to the car idling fine when the IACV is unplugged.

 

The challenge on a 91 accord is to figure out why the ECU is trying to compensate. On vehicles after 1996, and obd2 scanner would list the sensor values the ECU is reporting to give you some direction. The 91 accord doesn't have that ability. All you know is the ECU is adding too much air through the IAC, but you don't know why it is happening.

You haven't found the root cause for the high idle. The most common cause for a high idle is a vacuum leak. A smoke test with a smoke machine on the intake is the typical way a shop finds vaccum leaks.

 

wouldn't that high idle caused by a vacuum leak only make sense if it still idled high even with the IAC unplugged?

 

Considering just breaking down the whole throttle and intake assembly to inspect it but no real evidence of a vacuum leak kind of stops me from doing it...

 

You can try a spray bottle of water around the intake to listen for an idle change when the engine is running.

Does the engine idle get lower or higher as the engine heats up?

 

Warmup behavior: kinda long feel free to skip parts
Vid 2 is of unplugging IACV behavior Vid 3 is of unplugging vacuum line to MAP

 

Wow, what a clean car for a 91. Overall, the ECU is trying to control the idle through the IAC. The issue is surrounding the ECU. The ECU could be getting a bad signal(s), have a missing power/ground, the ECU hasn't learned the idle, or the ECU could be bad.

The speedometer not working won't cause this problem. Solve the idle problem first, then you can work on the speedometer. Most likely bad solder joint on the speedometer circuit board.

When you unplug any component, the ECU can jump to a default programming, so you can ignore the behavior after plugging something back in. It likely doesn't have any meaning.

Google 62SN700.pdf. This is a 93 shop manual you can download that will be helpful. I'll reference some pages below.

I tend to start with simple items, then more involved.

First turn the key to the II position, but don't try to start the car. Does the check engine light turn on for about two seconds to do a bulb check?

Next is to check G101 that is the main ground for the engine computer on page 23-17. Unbolt that ground, use sandpaper or a wire brush to clean the eyelet and bolt to ensure you have a good ground. It is near the cylinder head where one of the radiator hoses connects. You want to ensure the ECU has a good ground.

Next reset the ECU by pulling the backup fuse for 15 seconds. You can do this with the car off. Read through the idle setting on 11-71. The car is only up to operating temperature when the radiator fan turns on or the coolant gauge gets about 1/3 to below 1/2 on the cluster. You don't need an external tachometer, just use the gauge in the car. I'm wondering if your idle is at 620 rpm when warmed up and the IAC unplugged in the procedure. You can try the idle adjustment (I recall you aready did this, the thread is getting long) to see if you can get it set.

If this doesn't work, then next steps will be to see if the 5V reference is messed up. On page 11-11 VCC1 and VCC2 are tied together and branch out to various sensors, so they have a reference voltage and send the correct signal back to the ECU. One of multiple sensors from the same source could be shorted to ground and dropping the 5V reference signal. Easy to check, just unplug the map with the key in the II position (engine doesn't need to be running) and use a volt meter to measure voltage on each wire. One should be 5V. Repeat this test on the throttle position sensor and the EGR valve lift sensor.

Always reset the ECU after unplugging sensors to get it out of any default settings.

 

Check engine light comes on in the manner you've described.
I cleaned and sanded the ground, no difference.
Here is the idle when fully warmed up:


I will do the last part soon.

 

So that last picture is a normal idle. Does the car idle normally once you reach that 750 rpm in the last picture?