Do you have access to a compressor? If so you can use it to figure out where the compression is going. You have to rig something to connect the air supply hose to the spark plug hole on the bad cylinder. The, with that cylinder at TDC of the compression stroke, introduce compressed air into the cylinder. You will be able to hear the air entering the crankcase through the valve cover breather if you have a ring, piston ring land, or head gasket problem, entering the intake manifold from the throttlebody if you have a burned intake valve, or entering the exhaust manifold from the turbo downpipe if you have a burned exhaust valve.