I have a question about my 04 duramax 3500. What is the normal operating temperature of the 6.6? The fuel mileage went from 20+ mpg to 13 and i noticed the temp gauge seems to fluctuate form just under 100 degrees Celsius to just under 90 degrees Celsius. Not sure if this is normal .Doesn't seem to matter if the outside temp is -15c or + 10c. Thanks Greg.