Recently purchased this vehicle which seemed to run and drive great. I let it sit for a couple months and now I'm driving it again and I seem to have a problem. Everything was fine but the other day I had to merge into traffic using a lot of power and it started flashing the check engine light and felt down on power. Had a P0161 code... hmm, whatever, o2 sensor... Ignoring that one. It now does this (misfire and flashing CEL) every time I am on the highway and give it a lot of gas. Around town it's fine. Finally it did set a P0303 code. I changed the coil and it did not fix the problem. I changed the plug wire and it did not fix the problem. Finally I removed the spark plug and found this plug. it's covered in oil, wasn't even tight, and I don't even think it's the right spark plug... the part number shows it's for 2011+ GMs not 2001. odd. So I put in a cheap copper ACDelco R44LTSM and still, same issue. The engine sounds fine except when cold with medium application of gas pedal there is a sort of squeaking ticking sound, and when it's warm under high load (but not at medium load) it does the same noise. Engine oil looks fine and I haven't had to add any oil. My friend who works at a GM dealer looked at the picture of the plug and told me I need a new engine, probably my valve springs/lifters are toast which might have resulted in piston damage. I'm not ready to spend like $5K on replacing the engine so I would like to do some further diagnosis. I'm thinking compression test and pulling valve cover? Any input appreciated. Thanks in advance!