Hello everyone. I just did valve cover replacement job two weeks ago. I did manage to forget to torque down the lower intake plenum(I only finger tightened them and forgot all about it) which caused a MASSIVE vacuum leak that can be heard from miles away.

Anyway.. I reassembled everything which at this point I think my upper intake plenum gasket is no longer suitable to use because it was assembled for about 2 minutes of engine run + torque down. Because I am getting P0300 now.

I am experiencing misfire at high rpm range, unable to push past 5000rpm. But it is very subtle in lower rpm that the car doent even set a pending fault code.

I ran the code several times, all of them coming back P0300.
I decided to investigate further, I did some fuel trim analysis and this is what I got

At Idle
B1 Short term Fuel Trim 15~20%
B1 Long term Fuel Trim 10%
B2 Short term Fuel Trim 15~20%
B2 Long term Fuel Trim 10%

At 2500rpm
B1 Short term Fuel Trim 7%
B1 Long term Fuel Trim 6%
B2 Short term Fuel Trim 7%
B2 Long term Fuel Trim 6%

Should I replace the intake gasket once again? I checked all the vacuum lines, pcv connecting etc. Should I spray some WD-40 around the engine?