1990 4x4 4.0l Coolant leak from thermostat housing

[slickdaddylube]

I have a slow coolant leak from the thermostat housing/ top of water pump housing, the engine has trouble holding temp but usually runs too cold. I replaced the thermostat and it still didnt fix this issue, this might be a dumb question but the thermostat came with a rubber o-ring around it and my hayne manual said that it didnt require another gasket, would this cause the leak and coolant fluctuation, or could it be the water pump? thanks

 

[dvdswan]

Yes. The o-ring fits in a lip in the t-stat housing mating surface to the block which would be the seal.