Originally Posted by Volvo Diesel

For whatever it's worth, I experienced the above symptoms when my thermostat failed in the open position.

Were you ever able to ascertain if the mechanic actually replaced the thermostat? Or were his repairs limited to the monovalve?

A thermostat is not very expensive; I'd probably replace that and put new coolant in. Lots of info on this forum on both the proper coolant to use and how to properly "burp" the system after replacing the thermostat and refilling the coolant if you use the "search" feature.