What type of thermostat (there are several, the commonest is a rod)?
Thermostat and radiator
The mark 8 could have a bad thermostat, or a bad water pump. Check the coolant level, then the thermostat and water pump. If these are ok, check the cooling fan circuit.
Its too hot. Check the water pump or thermostat.
Check the coolant level. Check the thermostat, the rad. the fans, water pump. Many reasons.
First determine if the engine is really running hot if it is check to see if you are low on water, or could be the thermostat is not opening.The thermostat allows water to enter the engine block after it reaches a certain temp.
Take it out and dunk it in boiling hot water. If it opens it'a working.
Usually this is not the case. It looks like a bad thermostat, because no way the coolant is not getting hot, but a rusted up thermostat would not open for the coolant to circulate. If I were you I will change the thermostat first than check the water pump.
First check the water level. Low level will not get to the heater core. Change the thermostat. If you recently changed the thermostat, bleed the coolant system.
Someone turned up the thermostat, or the thermostat has failed. The thermostat tells the heater what temp to keep the water at.
Check the radiator fan operation.
i would check youre fan clutch if it is not electric. Then do a cylinder leak down est to check for a blown headgasket