i have a 2006 crew cab 4x4 long bed LBZ single rear wheel, I have recently had a problem with overheating as i climb grades towing approx 12000 pound toy hauler it seems to me that the fan clutch isn't coming on unless i use my manual override and drop it into 4 gear at witch time it kicks in and begins to bring the temp down. I've been told by other duramax owners that this is normal operating when towing heavy loads, I have a boost gauge and a egt gauge that i watch faithfully to make sure that turbo doesn't get over 1100 degrees. This truck has brand new head gaskets and a new turbo and is very well taken care of i do not beat it and i've had it since new. Can anyone shed some light on this