2005 M6800 always fired right off but then on a couple occaisions it would start up run a couple seconds and quit. On about the third restart it would continue to run just fine all day. In a few weeks it became more erratic about starting and finally would not fire at all. I changed the fuel filter and cleaned some crud out of the water separator and checked to be sure the white plastic shut off valve in top of separator was clean, Fuel pump worked and i verified that fuel was getting to the new filter, but no fuel made it from the filter to the injector pump when I cracked the bleed bolt on the side of the injector pump. I could feel and hear fuel cutoff solenoid working when ignition switch was turned on. After much frustration, pulled cutoff solenoid and it was obvious that solenoid rod was extending and shutting off the fuel when ignition switch reverted from start to run. Replaced the ignition switch and problem solved. I'm not much of a mechanical troubleshooter obviously, but thought this might help someone else avoid frustration.
