The mechanic installed a used 2004 3.5 in the Maxima after the old engine died and shortly after the new engine was installed it had a tremendous amount of power and the car ran great but the car had a shaking on the low end of 700 - 800 rpm while idling.
After much investigating my mechanic suspected that it may be the fuel injectors since all of the sparks were firing and the coils were good and so he replaced the injectors from the new engine with those of the old engine.
After doing so the engine seemed to be idling better but now it has for some reason lost the power on the higher end around 4000+ rpm and is choking up and at times simply cuts off above 4000 rpm (driving me crazy since although the car was intially trembling on the low end during idle, it ran great and had lots of power).
He then decided to remove the replaced injectors and reinstalled the old ones and for whatever reason is now idling great but continues to have the issue where it now loses power above 4000 rpm.
It also started to make a bit of a rattling noise when approaching the 4000 range from the top left of the engine on the passenger side right above the A/C compressor (but it's not the compressor). Can anyone please give me an idea or help me analyze the problem,
, my mechanic doesn't seem to know which way to go and I could really use the expertise of those on this forum in trying to troubleshoot the cause. I will gladly reply with possible contributing answers that would help identify any causes that may be tied to the issues, thanks. 