I have a feeling that the thermostat is not closing the port to the radiator, either because it isnt fitting properly or there is no gasket allowing water to flow past when it is in the closed position. I would lean in the direction of it not seating without the gasket but don't know without having it taken apart to look at.
The easiest way is to take temp readings after a 15 min warmup. There should be a signifigant diffrence between the head where the tstat housing bolts up and the top of the tstat near the upper raidiator hose.
Its important to go ahead and fix it. Not only will it not allow the heater to work properly but you could run into a lot of problems overheating especially in the summer. If the tstat is not properly closing off the bypass port when it is hot you will overheat when you are under any kind of load - like going up hills or pulling a small trailer.
The easiest way is to take temp readings after a 15 min warmup. There should be a signifigant diffrence between the head where the tstat housing bolts up and the top of the tstat near the upper raidiator hose.
Its important to go ahead and fix it. Not only will it not allow the heater to work properly but you could run into a lot of problems overheating especially in the summer. If the tstat is not properly closing off the bypass port when it is hot you will overheat when you are under any kind of load - like going up hills or pulling a small trailer.