new thermostat, why flucuating temperature?

[niterider]

i changed my thermostat to a ford unit with the BB in it. i get plenty of heat but the temperature doesn't stay constant? has the system just not purged itself yet?

 

[Nhaz]

If its going up slowly and dropping quickly and then going up slowly. Sounds like normal thermostat operation. Cold weather makes thermostat stay closed to keep the engine warm. then it will pop open and drop the engine temp rapidly and then it pops closed again.

Summer months it tends to stay in a half open position so you don't see the bouncing.

If its staying in the normal range I wouldn't worry about it.