It could be the heater control valve.
It works like this:
A vacuum-actuated water valve is used to control engine coolant flow to the front heater core. A second vacuum-actuated water valve is used to control engine coolant flow to the rear heater core, on all models with an optional rear heater.
When vacuum is applied, the water valve closes and coolant flow is bypassed back to the engine. When the valve is opened (no vacuum applied), the coolant flows through the heater core and back to the engine.
Vacuum to the front heater core water valve is controlled by the push button mode control switch. Vacuum to the rear heater core water valve is controlled by an auxiliary vacuum switch that is actuated by the temperature control lever.
The water valve cannot be repaired and, if faulty or damaged, it must be replaced.
...but not steaming hot. No heat coming out either. I pulled the water lines from the pump to the heater core. No fluid coming out of the line while idling. Hose may be plugged but unlikely. Question? Doe...