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I recently installed headers and a full exhausts from speed engineering. The O2 sensors were all installed back to the places that I found them. I’ve been getting P0300 random misfire codes and running quite rich. I understand that I’ll need to tune the PCM however it was quite extreme when I was driving home and the whole car was backfiring yesterday.
Today, I switched the coil packs. Did all the tests that I’ve read about. I had two exhaust tube running around 140 degrees Fahrenheit while the rest were at 200+.
I marked the coil packs and swapped sides. From the passenger side to the driver side and brought the ones that I thought were faulty to the front for easy access. I also pulled a plus and they’re completely black. See photo attached.
I know there is a way to test the coil packs. But it seems to do this after a while of driving. Is there any other tests I can do before tuning the car.
Spark plug wires are new from last year, the spark plugs are all new from last year. What I did realize was that before removing the cats I scanned the car and got the same code. So obviously the code has been there prior to installing headers.
My next steps would be pulling all the plugs cleaning them, tuning the car and potentially changing all 4 O2 sensors.
P0300 is so common after installing headers. It should be an easy fix. Or you’ll over look the easy stuff and still have the problem.
plugs: you could have damaged one during the install. Just replace them from a known supplier such as summit racing; get NGK TR55 copper tops they are cheap.
wires: same, could have damaged them during removal. Just replace them. Be sure they are firmly plugged.
be sure each coil pack plug are snuggly fastened.
next be sure you don’t have an exhaust leak. You could have damaged the header gaskets. Be sure O2s are fully seated. After checking, double check. Tighten **** down.
Cat delete requires downstream 02 simulators or tuned out.
You need a tune.
also. You need a tune. Can’t stress that enough. It’s going to run rich from the headers and cat delete.
good luck.
Last edited by Finkledbody; Yesterday at 02:20 PM.
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Headers LS1 running rich 
