What is this coolant system part?

[trace2race]

This is located over the transmission under the trunk area on passenger side. It is what burst when it overheated.

[JFP in PA]

If this is a Tiptronic transmission car (it is always a good idea to include the car year, model and how it is equipped in a post), that is the control valve for the transmission oil cooler.

[trace2race]

My bad 2001 Boxster S with automatic tranny. Thank you.

[Sapientoni]

It looks like a water cut off to the heater core.

[JFP in PA]

Originally Posted by Sapientoni

It looks like a water cut off to the heater core.

The heaters in these cars are a constant flow design, there is no water cut off valve. Temperature is controlled by moving air mixing doors in the heater box.