My heater problem

I've read through alot of threads about heater issues and got some good ideas and eliminated a few.
My heater never worked very well in the year and a half I've owned my 89 xj with ac.
At first the engine temp needle never moved very high and the heater blew barely luke warm. I replaced the thermostat and it worked great. It blew hot and the engine warmed up quick as a new vehicle does. It worked great for the next cold month. Then the weather warmed up for a few weeks and I didn't use the heater for a while.
Now it's cold and my heater is blowing cool again. When I move the temp control on the heater it is obviously colder when set to cold but the hot side isn't much better. RPM's seem too have no effect on it.
The blend door seems to be opening and closing properly.(you can hear it clunk shut.)
The engine temp seems to be where it should be.
The linkage on the heater control valve under the hood moves when I move the temp control on the ac control. Is it possible that the butterfly valve inside this housing is not opening even though the linkage moves?
Since it went from working great to poor, this is my theory. But I know it could be the heater core too.
Has anyone seen one of these valves go bad?

I was bypassing the heater core because it was leaking in my '89, and the control valve just fell apart as I was loosening the hose clamps to it. $30 later I had a new HCV and still no heat

yeah, after 20 years, the plastic is shot . . . .

dont buy the heater valve from autozone, the one they sell is allready broken 1/2 the time when you buy it.

Prime suspects are the heater control valve and a clogged heater core. Buy your heater control valve from Jeep if you replace it.

Once you replace the heater control value, if that doesn't work, disconnect both heater hoses and run a garden hose through them both ways. This fixed my 88'.