Have a 1998 mk IV ford fiesta. The coolant temp gauge on the dash does not work alas. Put a new heater coolant valve in as the old one just burst and now it blows incredibly hot even when blower on first setting plus the plastic casing v hot to touch. Should it be that hot?  At the MOT it was noticed tbat the engine seemed to be running hot. I also replaced the thermostat at the time. Fan works aftr half an hour idling. Any advice? Its the hcv really its so hot. Thanks anyone.

The heater control valve on these work by varying the open:close duty cycle so every now and then it'll let through another burst of hot water. If the target temperature is set high this will be increased to the point where, at max temp, it is constantly flowing. In all cases the water will be at engine operating temperature which, at 90c or so, will indeed be very hot to the touch. If it still blows hot water in a cold setting (allowing time foe the matrix to cool) then it suggests it is stuck open. Quite common for poor quality aftermarket valves to fail like this. It is a terrible design but has made Ford millions over the decades.