Sounds typical of a leaking water pump seal. Sometimes cooling system pressure pushes on the lip seal and it seals better. Sometimes pressure pushes coolant past the seal. Some leaking coolant can evaporate or blow off, and sometimes it collects in a puddle that you don't see until after the engine is stopped.
The easiest thing to do is to use a cooling system pressure tester to vary system pressure while watching the gauge and looking for leaking coolant. An alternative is to add a small bottle of dye, drive the truck for a few minutes, then shine a black light on the area. The dye will show up bright yellow. You just follow it to the source of the leak.
Tuesday, February 9th, 2010 AT 7:02 PM