2008 mercury mountaineer. heat will blow and then it will blow cold. Coolant is full and no leaks. It's overheated twice in the past week. Now high rpms at low speed, when heat is turned on or off.

All the classic symptoms of a cooling system full of air. I answered this question so many times, I turned it into a Fixya Tip. The problem is that the heater matrix is high in the vehicle dash and stays full of air unless coaxed out. Also, the heater control must be on full heat so coolant rushes through the matrix rather than trickling.

https://www.fixya.com/cars/r29872074-vehicle_cooling_system_no_heat

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Your radiator may be blocked

the air from the blower will be the same temperature as the coolant temperature. If you want it hotter you may have to get a thermostat that opens at a higher temperature or close the air circulation vent so that only cabin air is heated.and not fresh freezing cold air from outside.

You most likely have an air pocket in the cooling system. You'll need to bleed it. The coolant sensor is designed to monitor coolant not steam.

first make sure you have the radiator full of coolant. if it over heats after it is full of coolant then it is the thermostat. hope this helps.

2004 mercury mountaineer.heat problem
When turning ac knob to heat clicking sound like door opening and closing repeatedly sounds and air blows cold instead of hot.

If you are losing coolant but not leaking out, there are 2 other possibilities. Does it go intop the oilpan? Easy to verify by looking at dipstick. If no, then it is probably getting into the cylinders and going out the exhaust. Is the exhaust particularly moist? Causes for this would be a blown head gasket or cracked head. If you have more questions, please reply.

Have the thermostat changed.

if it has a viscous fan hub --replace it
if it has electric fans , check/ replace the coolant temperature sensor , fan relays or fan motors
have a compression test done to check for gasket or cracked head/s
over heating once a week is not constant so have the fault codes read to check for heated oxygen sensor problems altering the air / fuel mixture