Heater problems

[sandman]

I have a 2005 GMC Sierra Z-71. I am having a problem with the heater. Regardless if you use the heater in manual or automatic mode it doesn't seem to want to blow hot air. The motor is plenty hot. You can punch the accelerator and get the truck up to about 70-75 mph and the heater will start to blow hot air...for a little while...then it begins to cool back off and blow cold again. Anybody know how to fix this???

[03chevy]

have you checked the heater control valve?

The heater flow can be checked by feeling the hoses at the inlet and outlet to the heater core. If the heater has a control valve and it is open, both hoses should be warm. This indicates that coolant is flowing through the heater core. If the outlet hose is cold, there must be an obstruction in the heater core or control valve.

[03chevy]

here is how to replace it.

The heater control valve (sometimes called the water flow valve) controls the flow of coolant into the heater core from the engine.
Heater control valves are operated in three basic ways: by cable, thermostat, or vacuum. Cable-operated valves are controlled directly from the heater control lever on the dashboard.



Cable-operated heater valve used to control the flow of water or coolant through the heater core.
Thermostatically controlled valves feature a liquid-filled capillary tube located in the discharge air stream off the heater core that senses air temperature and modulates the flow of water to maintain a constant temperature regardless of engine speed or temperature. Most heater valves utilized on today's car are vacuum operated. These valves are normally located in the heater hose line or mounted directly in the engine block.
To replace a heater control valve:

• Refer to your vehicle's service manual to locate the valve.
• Drain or lower the fluid level in the cooling system.
• Unclamp, unbolt and unplug the valve.
• Reverse procedures to complete installation.

CAUTION

Identify the heater control valve carefully. It is easily confused with other parts.