Bought 2002 duramax new--problems began summer of 2010. Motor would overheat while pulling grades. I have Bully Dog that I run at "tow performance" or 2nd setting. Large exhaust, larger air filter. Also Pac Brake. Started w/ diesel mechanic first replacing thermostats, then because the transmission was overheating also, I put on a heavy duty trans cooler. Overheating problem persisted. Has 59,000 miles, thinking that the coolant may have gelled in the radiator from lack of use, replaced radiator and coolant. Overheating continued. Perhaps torque converter possibly locking up, had the transmission rebuilt. Problem persists. Replaced everything in the cooling system, pulled the heads, replaced head gaskets. There was sign of blow by but not enough to cause overheating. While heads were off, replaced water pump and housing. No help. Replaced fan clutch, waste gate for the turbo (thinking I was loosing pressure in the turbo). Had turbo system pressure tested to 30 psi held for 5 min. Had mechanic ride while pulling heavy load with his computer connected to the truck. Indicated fuel intake was proper levels for throttle requirements. Still overheating. No exhaust restrictions per the mechanic. Last test pull done engine temp went to 251 causing defueling, pyro 1251 degrees, transmission 255; fan clutch kicked in at 235. Turbo boost at 20 psi. Have had it evauated by 2 GMC dealers and 2 respected diesel mechanics. The problem has not been solved at the cost of $11,000. Any suggestions???? Please!!!!