CM570 compression in fuel

[jdrmx]

Been browsing this site for a while, so I registered as a member. Now I have a question nobody can answer. I'm working a signature 600 cm570. Runs horrid and rough with a heavy combustion knock and I have gobs of air (big bubbles) dumping out the return. I've leak tested the front and rear injector bank. No air bubbles at all from the metering actuator test fixture while cranking the engine with the starter. Swapped timing and metering actuators with no change in cylinder performance test and ran a cylinder performance test with with a diagnosis of #1 injector as the culprit. #3 is fueling 200% and #1 is 15% and the injector orings are like new with no visible damage to the injector sleeve. The suction side of the system has been eliminated by checking the fuel for air out of the test port under the front meter actuator according to the cummins procedure. These cheap pro diesel injectors that were installed are a couple weeks old and i've already replaced 3 of them due to combustion coming into the metering rail with the leak test. My question is can combustion enter the tip of the injector and get into the return fuel "any" other way if its not coming through the metering check valve into the metering rail or past the injector/sleeve seat past the orings? The other injector orings all are like new also.