I'm new to this forum and the Z. My girlfriend just bought a 2003 350z with 188559 miles on the body and around 100000 on the engine. The owners before us bought it with a blown engine and did a terrible job putting the new one in. With me being a mechanic me and my girlfriend decided to get it for a real steal at $5500 in good shape. It had the ses light on so we checked it out replaced the culprit (crank sensor) and after I reset the ecu another code (missfire in cylinder 5) we replaced the coil pack after I figured out that was the problem. After that it was running good but it would lose power at around 4000 RPM and make a hissing/sucking sound. Only code was a p0328 (knock sensor high input) so I had them clear it thinking it could have been from the coil pack still. Today I had it scanned again and got the same code along with p0301 (misfire in cylinder 1). This time it did not sound like it did before when the coil pack was bad. I have done some research and have narrowed it down to a few things: ECU, fuel pump, knock sensor, catalytic converter plugged up from running bad for so long, or the throttle body. It has all new spark plugs and it idles great. Any help would be appreciated. Thanks for everyone's input in advance. Car is automatic and has fresh gas and fresh oil change. So it's not that. Please help because this is starting to get over my head. I am very mechanically inclined I just don't have a good scan tool that can read me live data.

 

I would check exhaust, that sounds like a possible plugged up cat if it's fine at lower rpms and loads. Does it rev freely to redline in park/neutral?

 

Most of the time it does.

 

This morning I was looking around the engine and I noticed that the knock sensor was unhooked. I searched for half an hour trying to find the other plug from the 02 sensor harness and plugged it in to the correct plug. That improved the engine drastically. Still not up to par though. It has more power but it is still losing some at around 5000 rpm this time. It also makes a rattling sound at higher rpms. Could this be a valve floating? Or still something i noticed earlier?

 

Seems to run better when engine is cold.

 

Also got a p0300 yesterday. I noticed that it backfires when I let off the gas. We ordered some test pipes so we will see if that helps in a few days.

 

Has the ecu been flashed or had custom tune?

 

As far as I know it's completely stock.

 

The cats were both broken. Berk test pipes and problem solved.