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

[Blas]

The water level in your chrome expansion tank should be up to the divider inside the tank. Remove the cap (when cold) and stick your finger inside the tank, you will feel the hole in the baffle plate. Any extra coolant will be sent to your overflow tank (assuming you have one)
Smith’s gauges work or they don't. Accuracy is another story. The standard SPF temp gauges use capillary tubes.
A wildly reading gauge says you have just air in the intake manifold. (You hit steam then coolant hence the wild readings) You can try purging the system by opening the expansion tank (NOT when engine is hot) Start the engine, add fluid to expansion tank. The water temp sensor is usually in the front of the engine, look for a small brass tube wound with a steel wire. Loosen the brass fitting on the end of this brass tube and watch for air escaping along with a little coolant. When the air is gone you'll only get coolant. Tighten it back up. Having the front of the car higher than the back helps too when doing this.
Did you have the small hole(s) on your thermostat disk orientated up? If there are none on the thermostat disk, you can drill one. Careful with the thermostat orientation engine vs. radiator side when reinstalling the thermostat.