I just bought my 99 Silverado Z71 RCLB, 5.3L, automatic. Has 155k on her, and I think she's had a real hard life. I replaced the plugs and wires as well as running sea foam through the intake and feul to clean it up. Cyl 4 is misfiring consitantly and the oil pressure is unusually high, like 60 psi @ 2k rpms. Oil is mobil 1 high mileage less than 1k old.

Any suggestions would be greatly appreciated, I am fairly handy and would love to fix this myself. My buddy told me seafoam may have eaten the piston rings, is this a possibility?

I swapped plugs and wires and the misfire did not move, I plan to swap coils next. Thanks!