Hey Guys I have an 04 Neon with 49,000 miles on it. I have a custom exhaust with a header, no cat down to a glass pack with srt4 dual tips. Recently I noticed after i removed the magnaflow muffler and put on the glass pack the vehicle idles really really rough and the CEL comes on with a P0300 or 301 I think with a multiple cylinder misfire and cylinder 1 misfire. I believe its from insufficient back pressure from changing the muffler but Im not sure at this point. I changed the plugs, wires and coil pack. I have ngk plugs from modernperformance.com along with MSD coil pack and Granatelli wires all from modernperformance installed about 6 months ago. I think the engine code may be from the plugs being dirty and causing the misfire. Also alotta people say the problem is from the cat being removed and causing the engine to throw a code and rough idle. PLEASE HELP!! Ive had enough of this rough idle and feeling like i have a cam when Im at a stop light.

 

Have you cleaned and gapped the plugs yet...or better still changed them? You say that NGK's are in the car.....Platinum plugs or the Vpower? Platinum plugs in a Neon can and usually are bad news as they don't seem to last long. I use Vpowers with no problems gapped at .035. Since you took off the cat, what are you doing with the downstream O2 sensor.unplugged or neatly tied up somewhere? If that is the case go to Modernperformance.com and and buy an anti-fouler kit and have an exhaust shop install it in the downpipe about where it would be if the cat was there and install the 02 sensor. This fools the 02 sensor into thinking the cat is present. The P0300 is a multi cyl misfire and the P0301 is a #1 cyl misfire. If you have your old plug wires and if they are serviceable, install new plugs and the old wires after clearing the cel, to see if the code comes back. Then if it doesn't, install the new wires....if it comes back it could be your wires. My bet tho it's the 02 sensor. Just take things one step at a time and NOT in a random order.

 

I bought the NGK iridium plugs that were stock range but i think I shoulda bought the colder range cuz I have many bolts on to the motor (non turbo) and I drive it hard. i'll try to clean the plugs first then I'll go from there

 

whoa there cowboy! If you clean iridium plugs you can remove the iridium off the electrodes. There is very little of it there and easily removed. If you want to try colder plugs since you drive it hard, do that, otherwise you may ruin a good set of plugs. A colder heat range IS a good idea on a car that's driven hard, I did that with mine. I have an OBX shorty header, OBX cai, granatelli coil pack, magnacore wires, Mopar Stage 1 cam, Mopar Stage 1 clutch and other stuff, but it just supports these mods.