I had a mechanic put my 99civic on a machine, he said O2 sensors were bad, so instead of replacing sensors and the cat I went with an after market header. As I started this job I found immediately that the oem stock exhaust manifold had a crack in the center all the way around it, so I had to replace the header anyways. I left the O2 sensors off of the new header and the cat was connected to the oem exhaust manifold so I didnt replace the cat. I crank the car, purrs like a kitten, at first, I even took cables off battery to try and reset the computer. It did and ran for 5-10mins perfectly. Then the engine light starts flashing and the car starts to miss out really bad. I pull plugs and car is running very lean, white almost ashy look to the plug electrodes on 2 and on other two electrodes were dark and smelled of gas. my question is..........The computer still thinks it needs the O2's and the cat, so its giving bad commands or commands that it thinks are correct for air and gas flow...???? Correct?????? So my next question is this, if my assumption is correct on the first question. How do I get this resolved? I have friends who have done the same as I and they have had no problems except for a check engine light that stays on and a loss of some mpgs. Please some one help!!!!!!!!!!!!

 

Could it be the IAC Valve? If I do take valve off to clean or replace will I have to bleed cooling system also?