Hey guys,

I recently purchased a 2004 Monte Carlo SS (3.8L NA) and noticed a weird issue thats kinda got me stumped. The car runs very smooth for 380k km, and has been well maintained as a one owner car. The previous owner also went to the same mechanic as me who I know is reputable and does fantastic work.

The only issue with the car that I've noticed is that it throws a couple codes for the cat, which makes sense given the age and mileage on the car,.

I recently took it for a bit of a harder pull to really see what the car could do after getting the oil swapped and wheels aligned and the weirdest thing happened. At anything but full throttle the car acts like it should, but with my foot all the way down when the engine begins to climb past 4k rpm I notices that the power starts to drop and the rpm jumps around. then it throws the code for a misfire even though the engine is clearly running fine on all 6 cylinders.

We used a scan tool and did some pulls and it appears that the fuel trims are acting as the should, so I have a feeling it has to do something with the air system. My plan going forward to replace the exhaust to see if maybe my cat is clogged causing back flow, change the air filter as i dont know the last time that was changed, swapping the mass airflow sensor to see if maybe the last time it was changed the previous owner happened to install an aftermarket one compared to the OEM one. The last thing I planned to do was take a look at my throttle body and see if that possibly causing the issue

Tonight I have filled the engine full with premium to see if theres any gunk in the lines I could possiby clear out with it and im going to run some seafoam through it as well just to make sure theres nothing going on with the fuel system.

Any of you guys have experience with something like this or can point me in a right direction if im looking at all the wrong things?