"Crack the line" means loosen the nut that secures the line to the top of the injector, letting the fuel leak out instead of going to the injector. You might place a rag under there to catch the fuel. Do them one at a time, and retighten after observing the effect. If loosening the line doesn't make any difference in the idle, you know that injector wasn't doing anything. If it makes the idle worse, you know that injector was working. In a spark-ignition (gasoline) engine, you would disconnect the sparkplug wires one at a time. Steve