Your statement is correct. If there is something wrong you will get a really strange reading from one of your glow plugs. Read them and post the results Will quickly indicate what if anything and where to look. The way it was done in the past was to loosen each injector line nut at the injector one at a time a little until the fuel seeps out and try to find the one that makes no difference when loosened to the engine if any. Both ways just show you where to look for the problem basically. Plus inform you what cylinder it is. Glad you are getting an understanding. Well worth learning in my opinion.