Temperture fluctation - 2005 Ford Explorer Sport Trac

Generally the temperature fluctuates due to a leak in the cooling system.

The cooling system operates similar to a pressure cooker, with everything sealed, for each pound of pressure on the system, the engine can increase the temperature above the boiling point by 7 degrees without boiling. This is why when a hose breaks, the temperature gauge generally goes all the way up & you will hear boiling if you open the hood & inspect the area as it occurs.

If you lose a pound (more or less) of pressure, even with antifreeze, due to the pressure the system is under, the temperature will rise.

So, when there is even a pin hole sized leak, a gasket that occasionally leaks, a radiator cap that isn't actually holding the correct pressure-all of these things will cause the pressure to drop & an immediate change in the temperature making it hotter and as it re-seals itself, the temp will decrease again. There are a lot of particles floating through your cooling system and these particles sometimes will plug a leak until pressure pushes it out later.

Generally a leak is iindicated by the temp gauge rising higher than ever before. To resolve this, a pressure test will be necessary unless you replace the radiator cap & find it is no longer fluctuating.

Also, this doesn't have to happen each time you drive. Seals leak when they want to & that's all I can say about that. Usually they will reach a point where they will want to leak all of the time but finding them has to be done while they are in fact leaking.

good luck