PIA or not, Rob, I've gotta wonder about the condition of your heat exchanger
. That bad corrosion
at the cap suggests a nasty situation inside, which may interfere with proper cooling
. It's like finding rot
in a piece of wood - worthwhile digging further until you find good wood, rather than just treating the surface.
Sorry to be coming forth with a discouraging suggestion. It's your call, because it's your PIA.