Unless Ford did something different with the Zetec, the cap is only for the reservoir. It is pressurized, but there is no cap on the actual radiator. Do the fans kick in when you turn the AC on? I haven't had any cooling issues with the 'Tour, but my mother's Taurus has quite a few. The main culprit was the plastic reservoir tank was leaking slowly. That was a pain to figure out. It still loses coolant, but no problems. We have given up..
In your case, the coolant level is good? I would suggest flushing it out.
