It sounds as if you have a thermostat that is stuck open.

When you install the thermostat in put the large end of the unit towards the engine, i.e., the flat part should be facing our towards the top radiator hose. The bulb on the dack of the stat contains a liquid that expands when it get hot. As the liquid expands it pushes the center of the stat open and allows the hot water from the engine into the top of the radiator. If you watch your temp gauge as the engine heats up you will see it go past the normal temp and then fall back to the normal range. The falling back is an indication that the stat is working.

As was said you can check this out by putting the stat in a pan of boiling water. If you have a thermometer with a 250 deg. range you can tell at what temp the stat opens.