One way to temporarily solve the problem is to use a hose clamp designed to
pinch a hose closed and apply it to the tube near the valve.  The valve is
accessible after you remove the spare tire.  You can use a pair of snipenose
visegrips with the jaws covered in bicycle innertube rubber and use a ziptie
to hold the pliers in place.  The objective is to stop the flow of hot
coolant through the heat exchanger.   In my case the yahoo who installed the
coolant feed plumbed it backwards so the direction of flow tended to open
rather than close the poorly designed valve.

p