My 04 sierra is having a misfire problem, the engine is an 02 LQ9 6L out of an Escalade. Cylinder 4 is the one misfiring, and has low compression but it is still barley firing. Plug is not oily whenever I pulled it out but when I took the plug out and hooked an air compressor up, took the oil cap off and there was blowby. Coil, injector, plugs and wires are all good. Have a feeling it is a valve issue as the one head had a slight amount of rust In one valve seat (so i was told by my mechanic when he was doing the swap) but it runs fine still, misfire iant too noticeable when driving just want to finally get it fixed. Any help apreciated

 

You answered your question, valve job or replacement head,bummer,stuff happens