Originally Posted by kyden
you might have a blocked heater core.
as for the thermostat, does the engine come up to normal temp?
Ditto the above^^^.
If engine temp is OK it is either a restriction or the valve is not working, easy enough to check.
Once engine temp is up to normal, feel both the heater core hoses when heat is on, both hoses should be hot, if there is room check hose temp on both sides of valve, if one side is hooter/cooler then other side, replace the valve, [it's defective].
If heater core is plugged you may be able to flush it out, disconnect both heater core hoses and connect up a garden hose to the heater cores return side and turn on the water and flush out the core, you can even "break" stuff free by switching the garden hose back and forth between the inlet and outlet of the core. 94