Normally, the temp gauge in my 4-cylinder 1993 Dodge Caravan stays a half-division below the middle, once it warms up.
Recently, though, it sometimes goes all the way to hot and then back down to the middle, taking maybe 5 seconds to go up and then 5 seconds to go back down. This seems to mostly happen after the car has been sitting for a long time (like a week), and when it happens, it only happens once, usually around the time the engine gets warmed up.
I also noticed this weekend that it would frequently go up a division or so and then go back down again, again, taking maybe 5 seconds. It happened maybe 10 times in a 50-mile trip, often (but not always) when going uphill.
I’ve checked the coolant level – the reservoir is close to max. I also checked that there isn’t an air bubble by loosening the radiator cap.
Any idea what this is? Thermostat? Fan (or fan relay)? Water pump? Faulty temperature sensor? I don’t know enough about how the cooling system works to hazard a guess.