I have been slowly diagnosing (and changing good parts) on my 4.3 vortec since a misfire started in cylinder 4 a few months ago (code 0304, Jimmy has been in my garage since). I erroneously assumed it had dirty injector poppers and converted to the “new” MFI system. Didn’t fix the misfire. Actually, not only do I have a misfire, it now runs so rich I feel like I could put a dish under the exhaust and collect gas to reuse (j/k but the smell and blue smoke are obvious). I did a compression test and #4 is only capable of 90 psi. All the other cylinders were between 105-110. I know, not good all around, but i never had a problem until the misfire. Leak down test presented a leak in the intake valve (loud air/hissing could he heard from the throttle body). Inserted a bore scope into #4 and it seemed like I might have spotted a small crack on the intake valve. It looks like it seats well though. Looking at the valve springs, I can’t find anything broken, but could the intake spring just be weak? Or is there something else that could be wrong with the guide/seal that might not allow the valve to seat all the way? I’m kinda new to this engine diagnosing thing. Could really use (and appreciate!) some help. Can anyone offer some wisdom?

Intake valve #4