I had a stepper water temp gauge plumbed into the top hose and mine would cut in generally anything between 98degC-102degC. I've had them cut in as late as 108degC before. I would say anything above 110-115 is the danger area.
EDIT: Zetec engines inherently run warmer than most engines.
Last edited by pani_k; Nov 13, 2012 at 01:03 PM.