I am going to add some information, as I have been following the post. Before I would invest anymore time or money, I would have your cooling system checked for the presence of combustion gases. By and large, most Northstar cooling system problems are the result of internal leak, especially if there are no external leaks/low coolant. Often times the higher the load, the more super-heated combustion gases are introduced into the cooling system. What happens is the result of poor design. Through the extreme torque placed on the head bolts, and expansion and contraction of metals, the threads (aluminum) in the block end up failing. The repair involves engine disassembly, and steel thread inserts installed to replace the original aluminum ones. In twelve years, I have yet to see a repeat failure after the repair has been done, and done properly. Hope this helps.
Thursday, October 28th, 2010 AT 9:51 PM