I write this for the benefit of bors having fluctuating coolant temperature. There are certain issues that you should be aware. This is not one of those old engines where dancing temp.as long as it does not hit the red mark is fine.Temp fluctuation on these modern high technology engines has several consequence.
Coolant temperature for optimum engine operation is 90 dee. After start the engine gets up to this temp. as fast as possible and maintains this temp. steady through out it operation. Fluctuations of temp. during engine normal operation is abnormal. If anyone says as long as the temp. does not exceed 90 deg. it's normal or it's ok, is talking absolute nonsense. I will explain why!
During engine operation, tremendous heat is generated internally. Temp. indication provides an excellent monitoring parameter to the driver. The coolant sensor plays a crucial roll by continually delivering temp signals to the ECU which figures out if the engine is cold,warming up, normal temp. or overheat. Using these signals the ECU affects the operating strategy of the entire engine management system.There's a lot more signals that enter the ECU for engine management, it will be too long winded to discuss all. Let's keep it to coolant temperature sensor.
Many of the fuel, ignition, emissions and drivetrain functions handled by the ECU are affected by the engine's operating temperature. A different operating strategy is used when the engine is cold than when it is warm. This is done to improve cold driveability, idle quality and emissions. Cold/warm feedback controls the air/fuel mixture. The ECU may ignore the oxygen sensor rich/lean feedback signal until the coolant reaches a certain temperature. While the engine is cold, the ECU keep the fuel mixture rich to improve idle quality and cold driveability.
Therefore unsteady temp. signals can cause high fc and continued driving with this condition can lead to foul plugs. There's a host of other things that take place all based on signals from coolant temp. sensor, from cold/warm/overheat. Again it's too long to discuss all.
Don't take an unsteady temperature indication lightly!