Hi Everyone,
To start off my Z is a 2003 Touring model. I bought it off craigslist in hindsight not the greatest judgement call especially since the engine blew not even 25 minutes after i bought it and because it was no refunds i chose to just cut my losses and just towed it home which was a few hours away. A chunk of money and a month later I got the new engine installed and even then I still had a profound amount of issues the car had a clean title and only 44,000 miles on it when i purchased it the previous owner said it was used for drag racing not sure if that's true nor is it what i use the car for its my daily driver. I love the way it looks and sounds apart from the misfire. I put quite a bit of money into my car and i'm not really complaining i do love my car and its been a lot of fun learning about it and repairing it. I'm sure if i didn't love it I would've just given up after the list of repairs kept getting longer and longer. And now i have this P0300 code that's pending and stored that's lasted at least a year now.

Some background information on what I've done include I have replaced the O2 sensors, the coils, I have redone the wiring harness, changed the fuel injector O-rings, changed the spark plugs, changed the camshaft and crankshaft sensors, the battery, the air intake, I've upgraded the majority of the coolant lines with the Z1 Silicone ones and checked the vacuum lines for cracks and couldn't find anything like that, added new grounds and cleaned the original ones, replaced the thermostat, A/C compressor, Clutch setup and clutch lines, Changed the passenger side fuel pump. and a variety of other things.

With the help of this site and the service manual i was able to do the majority of the repairs and reduce the cost. Which was helpful since I have some distrust when it comes to shops (very bad experiences in the past) but when i ran out of ideas I caved in and ended up taking it to the dealership to try and solve this misfire issue $200 later they said it was an intake manifold leak that seemed to be the culprit and told me to strip it down past the intake runners and then reseal it. Which i did a few days after and the car still had a pending and stored P0300 code so i decided im better off asking for advice here because this has been my go to place for help. Especially when they told me in order for them to remove my ignition coils and check for oil down the spark plug tubes they would have to remove my intake plenum and wanted to charge me $1100 which i declined.
After I've replaced all those things the car runs great no oil leaks now, no stalling, no overheating, the car idles at around 1200 rpms with some up and down movement according to the scanner by about 20-60 rpms. It has always idled high even before the code came up. I don't have catalytic converters just test pipes with the proper O2 sensor locations. The CEL never blinks just a constant service engine soon but for the life of me I cannot figure this misfire thing out and i'm hoping its something obvious and simple that i'm missing and not something wrong with this new engine. I've checked every other post related to this code and tried it all to no avail, so as a final attempt I would really love to know if anyone has any thoughts or anything that could help me out. thank you.

 

Welcome and congratz.
With test pipes you have changed the way the car breathes. The airflow is outside what your ECU is expecting so it throws a code. Don't get me wrong, you may actually have misfires but you say it runs well. Make sure your intake is snug. The idle is high, might want to put a vacuum gauge on the intake and watch for fluctuations.

 

Thank you for the advice. I will definitely give that a try and find out if that is the issue or at the very least a contributing factor.

 

i would question the sanity of who you bought it from to use a red wire as what looks to be a ground

 

Thank you for the reply,
I actually will have to take the blame for that one the original set up had to be modified when the new engine was installed, but I bought the red ground wire from an auto parts store because my battery didn't seem too stable and I read on here adding more grounds could potentially improve that, so I went and bought it from the battery department and it was labeled as a grounding wire but hopefully its not causing a problem. It did seem to help although it could just be a placebo effect. A big issue on my battery that the original owner did add is that little red box that's above the positive terminal, I got stranded one night and it turned out that the little box if touched or moved causes my entire car to short out and the car only will power up if that box is snug and connected I don't know why it had to be added, but I just secured it and haven't had a problem since.

 

In one pic it looks like there is a 4g power wire. Is there still an aftermarket stereo system in the car?

 

Yes there are these two batteries on the passenger side compartment and then on the driver side I have 3 amps 1 for the subwoofers and then 1 for the rear speakers and 1 for the front speakers. The front door panels are custom made I would think. Since they can fit 2 mids on each side so i have 6 speakers in the front and then 4 in the rear plus two subs in the rear as well.

 

I gotta be the only one to notice, but those headlight covers GOTTA go
They look like

 

Thank you for the feedback. Also, you're right you gotta be the only one since no one has ever said that to me before. Most people say something about the shaved door handles. They might just look better in person.

 

Kinda have to go w Calvin on this one. "Eyelids" were the thing on '05s and back but those are... Interesting. Jk br0, it's your car, do what you want!

 

Well truth be told they have been on my car since I bought it. I never had a say and they seem as if they are a pain to get off no real indication they can easily be removed so they grew on me. And honestly they must look better in person since it was never mentioned before. But yes everyone will have their own personal taste and preferences. Nevertheless, feedback is always appreciated thank you.

 

Jotcb I don't like'm either but I doubt I'd wanna hear off topic nitpicking about minor cosmetics in my troubleshooting thread.

That said, P0300 has been the bane of my existence for the past 2 years. I've tested/sanded my plugs, replaced my coil packs, replaced camshaft position sensors/O2 Sensors and sprayed starter fluid around the plenum to check for vacuum leaks. Nothing has helped. It Idles normally, gets a little rough for 2 seconds and returns to normal. I'm subing this thread and will share any useful info I find.