Killing the fuel to the effected cylinder will stop the fire and prevent the exhaust from being shoved back up the intake. This would help get rid of a sputter caused by the reverse flow. Check the Exhaust lobe or follower!! Or don't listen it don't matter to me . Been there on several motors. If you say you checked everything you did what else could it be. If the exhaust valve doesn't open it will still show compression. It still draws air and fuel and it still seals off on compression stroke so it fires. Just that when the intake opens the exhaust goes out that way. Shut off the fuel and you don't get the explosion to cause the back flow.[/QU
Ok, I won't since you can't comprehend what I've written. Not killing any injectors, just going into kill test mode, which is open loop and commands a preset ignition timing/pulse width to stabilize RPM. Back out of the kill test mode into closed loop, the putter comes back and RPM drops. A mechanical malfunction would be apparent regardless of computer mode, not to mention the fact I've replaced enough flat cams in Chevy small blocks to recognize the symptoms.