Why does your engine temperature fluctuate?

Answer 1

The engine temperature fluctuates some what because the thermostat is opening and closing all the time to maintain the proper temperature, the cooling fans both mechanical clutch types and electric do not always run at full speed and in the case of electrical fans only run when the engine is getting to hot. Most electrical fans will turn on at about 220-230 degrees and off at around 200-210 so the engine will constantly be getting hotter and colder when the vehicle is not moving but should be more consistent when cruising since the fans are not required.

Temperature fluctuations can also be caused by engine cooling system failures like a

failing head gasket which will pressurise the cooling system and halt coolant flow causing the engine to overheat. If your engine is not overheating and your heater works fine no excessive white smoke out of the tailpipe and you have normal fluctuations then you shouldn't be concerned about it.

You may also be low on coolant from a leak in the system.