I have an 89 GMC Pickup, the heat blows cool. Electronic controls, new heater core, thermostat and water pump.
The heat will get warm if you are parked and have your foot on the gas or if you are driving but once you stop and let it idle it will go back to cool. The thermostat is installed correctly and the heater core gets hot on both the inlet and outlet sides.
If the temperature control valve, which controls the water flow through the heater core is stuck or partially clogged it can cause that. If the flow valve is O.K. then your water pump isn't circulating enough water at engine idle. With low water flow
