Originally Posted by Revs2.9k

I wanted to share my thought process for anyone experiencing similar issues.

We immediately started investigating the immobilizer, as it's often recommended as the problem, especially since the key wasn't original to the car. I have a close mechanic friend with advanced Snap-on scanners and a blank second key from when I bought the car. The first thing we did was cut and program a brand new key. We also tested the immobilizer unit with a voltage meter, and it seemed to be functioning correctly based on information from Identifix (I believe that's the name of the software).

We quickly ruled out the immobilizer and determined the issue was mechanical, not electrical. So, I continued digging!

A car needs three things: air, fuel, and spark. Spark is easy to check, so I started there, inspecting the coil packs and plugs. These were perfectly fine and working properly. Since I had recently sucked up some water, I decided to perform a compression test, which came back normal across the board. As a last resort, I took the car to a local performance mechanic for a deeper look. I was out of time and couldn't keep messing with it. After catching him up to speed, he used a borescope to check the intake manifold, cylinder chambers, and everything else. He found a considerable amount of liquid built up in the intake manifold. We removed, cleaned, and reinstalled it, but the issue reoccurred about two minutes later. He checked again and saw the liquid build-up. A quick Google search showed that the only way liquid would get back into the intake is through the IAC valve or Throttle Body, as both systems are coolant-fed (a design flaw, in my opinion).

Based on this, we decided to investigate the IAC first. The amount of coolant that came out was astonishing. Upon inspecting the IAC, we found a crack between the coolant passage and air passage. We determined this was likely the culprit and ordered a new one from Amazon for $40. Unfortunately, it arrived broken on the coolant line. We installed it and did a coolant bypass for the IAC for testing purposes, but the issue persisted. We then performed a fuel pressure test, assuming the new IAC was fully functional. After chasing our tails, we concluded it had to be the IAC. I ordered a genuine Honda-branded IAC from eBay for $85, which took a week to arrive. I installed it, but the issue persisted.

Being more familiar with newer cars with advanced technology (I am a software expert for my day job), I started researching. It turns out there's a reset procedure for the ECU that needs to be completed whenever you disconnect the battery to help it relearn the car's conditions and sensors.

The procedure is relatively simple, and here it is:

1. Turn off everything in the car completely (radio, AC, etc.).
2. Disconnect the battery and zip-tie the positive and negative cables for 5 minutes.
3. Reconnect the battery cables to the battery.
4. Start the car and rev it to 3,000 RPM until the radiator fans kick on.
5. Do not shut off the car; let it sit and idle for 5 minutes without touching anything. Do not give it any gas.
6. Take the car for a very spirited drive for some time.

I followed all the steps above, but the car still had a surging effect, and a new P0505 code popped up. We removed and reinstalled the new IAC, ensuring everything was sealed and connected properly. The surging continued, and at this point, the car had been running and driving for about 10 minutes.

No one on the forums mentioned how long it takes for the car to relearn the sensors, humidity, etc. It took a solid 30 minutes of hard, spirited driving for the car to return completely to normal after all the steps mentioned above. Ever since replacing it with a genuine Honda IAC and completing the relearn procedure, there have been no issues with the car.