I have also noticed a difference in the igniter spark plugs that seemed to help on my truck. My M543A2 seemed to be intermittent on igniting the atomized fuel. I disassembled and cleaned the nozzle assembly, cleaned the little felt filters in it, tested the spray pattern on it, and found all in order. The spark plug also had plenty of fire. There have been comments made that trucks with the turbo intake will not or do not consistently ignite the preheat fuel spray because the air horn is different in design. Because of this, I looked at the difference, and can see the plug is possibly not as favorably located in the turbo air elbow. The spark plug that was in my M543A2 was of what I will call conventional design with the electrodes in a visually "normal" location. I went to a turbo parts truck and took the igniter spark plug out of it. I immediately saw the electrodes on this plug extended about 3/4' down from the end of the plug. I put that plug in my engine, and the heater lights every time now. It apparently got the spark down into the spray pattern for more dependable ignition. Has anyone else seen this difference in igniter plugs? I will have to look at the part number difference on them. I certainly recommend the extrended reach plug.