I had a 00 honda civic si and after i isntalled intake header exhaust catbak adn skunk2 intake manifold my car started idling bad it even threw a bad IAC code. I replaced the IAC and it still did it. I took it to a local shop and they fixed the guy said he fixed my fuel injection setup with pgm. I sold the car a year later.
Anyhow I recently started working at a local Honda delership as a llube tech, making my way up as a trainee. Today a similar problem ocurred. probably even identical, maybe someone can help because it stumped even some of our lead techs.

A 00 Civic si pulled up with a chek engine light on, IAC problem. B16a2 engine: intake header exhaust skunk2 IM bored out TB. WE replaced the IAC and it worked for a couple hours, then he came bak same problem. It idled funny and when lightly pressed on the throttle the idle jumped back and forth hesitating at times. Then we shut the car off and started it again. Problem went away after a couple minutes it came back again. We cheked for vacuum leaks, none found. Now does anyone have a clue wa else to lok for?? PCM?? TPS??? please help!!

 

i had that problem with that intake manifold. my friend had it on his motor and it did that, and he sold it to me and handed off the problem to me. his was back to normal with the stock b16 IM.

 

Man I am goin throught the same thing right now no cel light though just replace iacv now i am going to replace tps and fitv.good luck if you find out something pls. post this is kicking my a$$.