Okay, you are saying the engine runs okay but you have no heat, but every couple days the temperature gauge goes up high and suddenly the heat starts working and the gauge goes back down. And you have added coolant with no change. Correct?
First how hot has it gotten? Just higher than normal? Hot enough to boil over?
From the description it sounds like an air pocket in the engine that is blocking coolant flow. The question is how did it get there.
Is the carpet damp and sweet smelling? Any unusual puddles under the car? A lot of white smoke out the exhaust?
I would do or have a cooling system pressure test done to determine if you have a leak that is causing you to lose coolant or worse something like a head gasket that is starting to fail. It would also show if something like a radiator tank was cracked or the heater core is leaking. (you can have a shop do it or visit a store like AutoZone and rent/borrow the tool to do it yourself)
If it passes the test, do a coolant flush and refill and change the thermostat, it could be sticking or starting to fail. While the coolant is out is a good time to change it.
Wednesday, January 4th, 2017 AT 6:40 PM