You may have trapped air in the cooling system or the heater core may be partially plugged up. Engine coolant is delivered to the heater core through two heater hoses. With the engine idling at normal operating temperature, set the temperature control knob in the full hot position, the mode control switch knob in the floor heat position, and the blower motor switch knob in the highest speed position. Using a test thermometer, check the temperature of the air being discharged at the HVAC housing floor outlets.
Ambient Air Temperature 15.5° C (60° F) 21.1° C (70° F) 26.6° C (80° F) 32.2° C (90° F)
Minimum Air Temperature at Floor Outlet 52.2° C (126° F) 56.1° C (133° F) 59.4° C (139° F) 62.2° C (144° F)
If the floor outlet air temperature is too low, Both of the heater hoses should be hot to the touch. The coolant return heater hose should be slightly cooler than the coolant supply heater hose. If the return hose is much cooler than the supply hose, locate and repair the engine coolant flow obstruction in the cooling system.
OBSTRUCTED COOLANT FLOW
Possible locations or causes of obstructed coolant flow: Pinched or kinked heater hoses. Improper heater hose routing. Plugged heater hoses or supply and return ports at the cooling system connections. A plugged heater core. If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.
Possible locations or causes of insufficient heat: An obstructed cowl air intake. Obstructed heater system outlets. A blend door not functioning properly.
If the heater outlet air temperature cannot be adjusted with the temperature control knob on the A/C Heater control panel, the following could require service: The A/C Heater control. The blend door actuator.
The blend door. Improper engine coolant temperature.
I have the same issue as mcebbie. This also applies to the AC in the summer. It blows air but when driving highway speeds, I finally get cool air. Same thing with the heat. No heat till Im driving on the highway. I notice that while driving, there seems to be a high idle noise. When i turn the heater or the AC on, the high idle sound goes away. What may this be? 1998 Dodge Ram Van B1500 V6