As stated in title, I am having a very hard time trying to pinpoint the cause of an idle surge (1000-1600 RPM when engine hot). Here's what I've done:

- IACV replaced (item 13 in diagram)
- FITV cleaned, replaced (item 17 in diagram)
- coolant replaced, bled of air
- ECU reset (pulled hazard and ECU fuse while engine running)
- Checked for obvious vacuum leaks

If you take the main black plastic intake hose off, the engine runs no different; if you look directly into the throttle body, there are two ports, like described here (https://www.hondaaccordforum.com/for...-2-2-m-t-6354/). One at 8 o'clock, and one at 10 o'clock. blocking Mr. 10 o'clock doesn't seem to do much although there is a small amount of suction there. Blocking Mr. 10 o'clock stalls the engine and has a strong vacuum. Additionally I removed the Air Boost Valve (item 8 in the attached diagram and blocked both holes on the intake manifold and the idle continued. Also, I think it is noteworthy that the issue only manifests when the engine is hot.

I have tried the common fixes like messing with the FITV and nothing seems to work. Could anyone help me with some more uncommon troubleshooting here?

 

Try cleaning both sides of the throttle plate and the bore near the plate. Carbon can build up and prevent the throttle plate from sealing. A rag soaked with some carb cleaner removes the buildup pretty easily.

 

Just cleaned it, a small amount of carbon came off but nothing significant. I do have an update however: Unplugging or blocking the IACV results in the surge stopping. When the IACV is plugged in, I can feel the solenoid triggering when the idle gets low and immediately a surge begins.It's a brand new IACV and I also tried adjusting the screw on it to no avail. What does this mean?

 

It means the engine computer is adjusting the air flow into the engine for some reason. The hard part for a pre-1996 car is why, because you can't easily hook up a scanner to figure out what the engine computer is seeing from various sensors.

You did your homework and did some of the checks, but I want to follow-up on some of them.

How did you bleed the air from the cooling system?

Did you pull the 7.5 amp backup/radio fuse in the engine bay fuse box to reset the ECU? You only mentioned the ECU and the hazard fuse.

 

I bled the air from the cooling system by using a funnel that sealed on the top of the radiator with the car not running. Once the car stopped drinking, I opened the bleeder on the thermostat until coolant came out. I then started the car and squeezed the upper radiator hose until it stopped drinking once more. Again I shut off the car and bled the bleeder valve on the thermostat. I left the car overnight with the funnel still on the radiator. In the morning it had drunk a little more. I capped it off at that moment and filled the overflow tank halfway.

I just pulled those fuses, like you said - While the car is running I pulled the backup/radio fuse, the hazard fuse, and then the ECU fuse. I then took out the key and plugged them back in. The issue remains.

For more information on the idle behavior, the car idles at about 1600 RPM while warming up, and then surges back and forth between 1000 to about 2000.

 

To update, I unplugged both plugs that lead to the box on the firewall that contains the MAP sensor and the idle behavior changed when I did that, the surges got far more frequent and lower in intensity. Could this indicate a bad MAP sensor?

 

Can you post the wire colors that go to the map sensor connector and the throttle position sensor? Those connectors are identical and can be accidentally swapped.

 

Here are the connectors that go to the box I was talking about and the second picture is the TPS

 

The wire colors are hard to see, but I think they are in the correct place. The TPS wire colors should be yel/wht, red/blk, and grn/wht. The MAP should be red/wht, wht/blu, and blu/wht. Just pull back the plastic a bit to verify.

Any other recent work on the car? TPS replacement, or adjusting of the main idle screw?

 

TPS has probably been replaced, I can tell since that gasket looks very new. Car is new to me so I am not sure. The main idle screw has been messed with as well, I can tell since it's all chowdered up. any thoughts on the MAP?