PX9 Knocking and black smoke

2013 T370 Ser# 407056. PX9 300 # 73550812
Engine at idle would periodically start skipping ( misfiring ), knocking like a bad rod bearing and the piston is hitting the head. Some white-gray smoke at times but usually black. Then runs smooth, clears up and repeat.
Narrowed it down to #4 and replaced injector. Is runs the same.
Misfire is pretty steady around 1300 but never happens around 1900 RPM.
Any ideas, thoughts and or experience appreciated.

 

Checked harnesses between ECM and rocker cover as well as rocker cover to injectors with OHM meter, OK. Powered a headlight through the same wires,OK. Swapped the injector harnesses, each does two cylinders, with no change. Checked all power and grounds we could find with no faults. Valve clearance is OK. Made a compression gauge and tested #4 and #1 while cranking. #4 was 400 and #1 was 410. Not 100% sure how accurate that old 1500 Lb gauge is but did not see 10 lbs as a deal breaker.
Thanking it is an ECM problem.

 

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