Loud knock, injector?

[OBSboy]

Since ive owned my 1997 f350 its had a knock in the engine. Theres no problem in power but fuel economy has been less then great and it starts real hard cold, and it also pukes blue smoke when starting. Its coming from the passanger side in the back of the engine compartment. I need to know the best way to diagnose which injector it is and if I can just change out the one injector or if I have to change out all eight. any advice will help thanks alot.

[OnTheBio]

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