To scotpa, Perhaps that's true but this is where my ignorance of the cummins engines shows through. In reading the service manual that is not what it says based on my understanding of what I read. Not saying I'm right, not saying the book is correct. But due to my lack of familiarity with these, I do use the FSM's as a guide. Evidence from JJdiese1990 also suggests that I'm wrong, and tribal knowledge on here is right as he ran his without a thermostat without any problems. However replacing the thermostat did cure the problem. See attached pic of the bad thermostat on the left and the new replacement on the right in an earlier post. Thanks for the reply.