First of all, i will tell you i just spend the better half of the morning reading mirfire post with little to no result. Current mods, aem intake, invidia header and Test pipe, tanabe exhaust.

I had a DC header for the longest time, i didnt like it very much so i went out and got a invidia header and TP. I installed it, and that was a pain within itself, and the car ran great till about 1000 miles.

After about 1000miles with the header on, i got the misfire codes, p0301-4 and random mirfire code. So i was like oh well ill just reset the ecu and see what happens. I reset and nothing, till about 800 miles! I am like what the hell, same codes. Change the plugs gap to 40, and the same codes popped at about 100 miles!

I dont understand what is going on, i checked for leaks and what not or anything to do with the install of the part and came up with nothing. This has been going on for months on end, ill reset the ecu and sometimes the codes will pop in 40miles sometimes they will pop at 300+ miles. Just doesnt add up.

Car runs fine till the code is about to pop, bogging at low rpms (-3k) but runs fine on the top end of rpms. Slightly rough idle also. Then the code will pop and the car will still run like that till reset, then it will be fine till it feels like popping codes agian!

I have no idea what is going on, ideas? Maybe i damaged an O2 sensor? I dont know that is all i could come up with. Help me with my headache!

 

just throwing it out there it may not even be header/exhaust related, check the coil packs maybe one isnt connected all the way or maybe you got a bad one. just an idea.

 

ive had a problem when i had codes for misfiring cylinders 1 2 and 4. i changed the coilpacks and spark plugs, after a couple hundred miles same proble. there is actuaLLY A recall on some cars , this is a common problem. im not even sure if it had anything to do with ur headers. my car only had 35k when this started happening. after i reset the comp the light hasnt been on yet. check ur sparkplugs

 

Well, the way the computer detects a misfire is through the crankshaft position sensor. if the car runs rough then the crank would spin erratically and that would be detected by the CPS.
My suggestion is to do a tuneup, but most important, tune the car for the current mods you have, all of them combined are enough to alter the volumetric characteristics of the engine so the computer is like WTF is going on?! and throws those codes that you get....