ANother puddle....biggest one yet

[Pdwight]

Wife and I took the Boxster out to diner and a 10 mile drive afterwards ...perfect weather and evening. After parking the car I came back out to get something and noticed a large puddle by the back tire and running away from the car (driveway on a hill). I popped the rear trunk and noticed a lot of condensation where the water was escaping. I grabbed some bottled water and topped it off....would up taking 1/2 gal of distilled water with the engine running. It never ran hot. I know I need to replace the water pump....already have a new one along with a lower temp thermostat.....but I am doing this as a preemptive maintenance measure since my car is at 80K miles. Could this be as simple as a bad expansion tank cap ??