Thanks, Gerald. That makes sense.
Your answer also explains something else I've wondered about. That is, when I come into a park with gravel or dirt roads after travelling some distance on paved roads or a freeway, the fan kicks up clouds of dust on the passenger side (fan is on the driver's side). It's embarassing because I can't prevent it, and it probably annoys the devil out of the people in the park that I have to drive by (sorry guys). However, when I leave in the am, there's no dust. So you've explained why- in the am the engine is cold and the fan is turning more slowly, but entering the park........WOW!
Thanks again, and BTW, it's definitely a Cummins 8.3.
Andy