Hi: I'm a new guy on the block. We (my son and I) just purchased a "new to us" 1998 GMC 2500 6.5 turbo diesel originally owned and operated by VDOT. We seem to be the 3rd owners since it was state owned. We bought it at a bargain price because of the loud knock and the code message that No. 6 cylinder was not working. Thought it might be an injector but with all new injectors have now been able to hear a very loud knock in No. 6. The body of the truck is in excellent shape for it's year, but the engine has 396,752 miles. Should we tear it down and rebuild it or just worry about the No. 6 cylinder?