a compression test won't exactly tell you valve guide wear . . . but it'll tell you if the piston rings are starting to fail, or if you have a burnt valve, valve seat, or possible major damage to the heads. Even more of a probability if the only DTC that's getting thrown is a P0300. There's something causing the PCM to flag a misfire, but the PCM doesn't see anything wrong with any of the input/output controls to flag a specific cylinder.
I just had a 2000 Cadillac Escalade with the vortec 350 that was throwing a P0300 - #6 cylinder was only pumping to 60psi on a compression test . . . loosened both rockers (to close the valves), and put shop air pressure on the cylinder, and she was blowing out the intake . . . yanked the heads off, and it turned out to have a torched valve seat.
I just had a 2000 Cadillac Escalade with the vortec 350 that was throwing a P0300 - #6 cylinder was only pumping to 60psi on a compression test . . . loosened both rockers (to close the valves), and put shop air pressure on the cylinder, and she was blowing out the intake . . . yanked the heads off, and it turned out to have a torched valve seat.
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