Originally Posted by
fordsd350
Not an expert, but I second the idler pulley.
Your description sounds about like what I had happening. Turned out to be the idler pulley. I believe there's a TSB out there for the 6.4L concerning this problem. Sorry, don't know the number.
Not that it's cold outside yet (although it was in the 40's in Colorado Springs this morning on the way to the airport) but maybe this is the TSB?
https://www.ford-trucks.com/forums/8...ml#post7098031