You probably have trapped air in the system. When the air gets on the temp sensor it goes hi and then when the coolant hits the temp sensor it goes lower. With a COLD engine remove the radiator cap, and start the engine and let it idle and keep topping off the coolant level in the radiator. MAke sure you see coolant moving or flowing inside the radiator once the thermostat opens. which is usually between 160 and 200 degrees. Once you get the air out of the system, the coolant flowing across the temp sensor will be more consistent and thi should take care of your problem.
on the water pump. follow the top hose from your radiator. it attaches to the block at the thermostat housing.
right below your thermostat housing
Either the thermostat is not working properly or the temperature sensor is displaying wrong on your gauge.
7.5 -10
Just follow the lower / bottom radiator hose to the engine and you will see where it clamps on a housing that has 3 bolts that hold it onto the water pump. The thermostat is inside of that housing.
sounds like you installed new thermostat upside down if that's not the problem, you have wrong or faulty new thermostat to test thermostat out of vehicle, place in pot of boiling water and watch to see if thermostat opens
if its a305,or 350 it is located intake by the thermostat housing.or in the t-stat housing
I can't remember exactly, but it's on the intake manifold side of things. It is be pretty obviously threaded into a water passage if you look around by where the thermostat is.
The thermostat controls only the temperature of the water.
You're Thermostat may possibly be stuck in the closed position, therefore not allowing coolant (water/antifreeze) to pass through to the engine. Basically check your thermostat is working.
The thermostat is inside of the water inlet on top of the motor. Disconnect the top water hose, then unbolt the water inlet. The thermostat can then be changed.
maybe try flushing the cooling system, or replace temp sending unit.