Overheating or bad gauge?

[timsullivan]

Bob: My guess would be a bad sending unit or gauge as the sending units are very sensitive and can be damaged quite easily. I would make sure the cooling system level in the chrome expansion tank mounted in front of your engine is at the mid point of the tank with the engine cool. If the coolant level is low or not showing in the expansion tank the that clearly could cause a bouncing temp reading. I would also check the connection for the water temp sending unit which on your 408 is most likely directly behind the distributor, on the intake manifold. If the line leading to the sending unit was properly installed it should have a large loop in it such that the line extends straight upward from the sending unit at least 3 or 4 inches. If it doesn't have the loop and was installed with a sharper bend at the water temp sending unit the line may be damaged.

I wouldn't worry about the lack of a "puke tank" to catch cooling system overflow. If the coolant level is maintained at the mid point in the expansion tank, when cool, your system will rarely throw out excess coolant when the car is warmed up.

-Tim