Reading the glow plugs is the simple thing to do. It gets you clear of or involved with the fundemental problem the quickest or most accurate way possible. You either have all cylinders preforming properly or you do not. It is not likely the rack bolt unless the spring packed it in all of a sudden in my opinion from your description. I think it should just grow weaker with age usually. Not a sudden change. But then nothing is impossible. Also the gentleman that mentioned poor fuel lines should be considered. Both from a perspective of him having had simular symptoms in the past and your observance of a larger than normal bubble in the pre filter. I would not even change the injectors out until there was an indication that perhaps one cylinder was reading different than the others by a wide margin. Also the smoking is not an indication of fuel starvation itself but strange things occur that do not make common sense from time to time. Even something like the head gasket failing between cylinders would be indicated. Yes reading the voltage between the harness plug and a ground on the cylinder head would tell you if all was well or not. The beauty of this approach is it's basically easy and might eliminate a lot of head scratching. Very good at elimination or narrowing down problems rather than excess work or expenditure. Too many cases of shotgunning service aproaches in the past. Thats also the primary reason for the emergence of this system as I have read over and over of people with poor idles that are lived with as there was previously no way to cheaply narrow things down.Or required skills that can only be aquired by a working mechanic to be definative.