if the switch changes nothing given your circumstances the only thing i can think of is a cylinder wo compression. compression can be lost by a broken piston or rings, a broken valve or valve spring a stuck valve or a bad head gasket. of those, bad rings dont usually come on suddenly so i would not think that way. the hole in the piston seems remote. a broken piston is a possibility. i would suspect first a stuck valve, then broken spring, then broken valve. the pistons and rings would be last on my list. it appears that you have eleminated all other possibilities by your investigation already. good luck.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.