I've noticed my engine fan will run when I start up the truck and will continue to run until about 3/4 mile down the road when I can hear the fan shut off. This is even when it has sat all night and ambient temp is on the 40's. I can understand why it runs in warmer weather but not cold.

 

Mine does it if I have the defrost on seems like. Not for sure why?

 

There's a viscous fluid in the 2nd generation fan clutch. It gets thicker when the truck sits out in cold weather, sufficiently so that it takes 1/2 to 1 mile for it to warm up enough to disengage the fan even though the bimetal strip isn't calling for the fan to be running.

Your 4.5 has an electrically operated fan clutch. It runs when the defrost is on because the defrost mode runs the A/C compressor for dehumidification.

Rusty

 

Thanks for the info.