Thank you Tazza, Eventually had diesel mechanic out and the short of it is - loosened each fuel injector line to each injector and at #3 the engine ran smooth again without black smoke. Removed all 4 injectors and had them tested at injector shop - all spray good but all right at low range on pressure 2408psi. Tech advise it might be low cylinder pressure seeing that injectors are acceptable still and recommend a pressure test on #3 - suggests blown head gasket, rings, etc?? Is this the acceptable line of thinking or should we be looking to other solutions or process of elimination. P.S Diesel mechanic seems really knowledgeable but was to reach out to someone with more experience when we found all injectors to be ok.