[s928s]

Sounds like a possible head gasket, ring or head/block issue. I would check the compression on each cylinder without the plugs. Next check the compression "wet" ( add a small mount of oil to cylinder) if the weak cylinder improves with the "wet " test it is piston or ring related. If the compression does not improve it is upper end related. I would then pull the head and inspect gasket surface and valve train. How is the timing belt, i would suggest pulling the cover and checking the timing marks if off by more than 2 teeth its going to have to come apart anyway !!