2008 GMC Sierra Heater blowing cold air

[Modesto Cartagena]

Good Day,

 

  Had to use my heater on my GMC 2008 Sierra 1500 and its blowing cold air out through the top vent and even on auto?  Do i need to replace blower motor resistor?  The Thermostat was replaces a few years ago.

 

Mo

[richard wysong]

if your temp gauge reads normal its either a plugged heater core or the motor for the temp blend door is ng. thats common. to check the heater core both heater hoses should be close to the same temp, if 1 is noticably cooler than the other reverse flush the heater core

[dna9656]

I agree with Richard. I hope this fixes it; changing the heater core is a big job if it's anything like my 06 Avalanche.

[Supreme Pizza]

On 11/20/2021 at 9:58 AM, dna9656 said:

I agree with Richard. I hope this fixes it; changing the heater core is a big job if it's anything like my 06 Avalanche.

 

If and when my heater core goes bad, I plan to go through the firewall, rather than pulling the dash.

 

For example, something like this:

 

1.  

[dna9656]

Was this covered by a goop sealed cover piece? What about all ther A/C stuff
? Did you have to recover the refrigerant?

[Supreme Pizza]

1 hour ago, dna9656 said:

Was this covered by a goop sealed cover piece? What about all ther A/C stuff
? Did you have to recover the refrigerant?

 

 

Are you referring to the picture of the hole in the firewall?

[Kiesel]

Modesto,

 

Having same issue on my 2001 Silverado. Were you able to back flush it? How'd that go?

I am replacing water pump and figured I could try this before digging around under the dash...

[richard wysong]

they make a tool for flushing the heater, it may be made by KD. it goes on the end of a garden hose and has an air fitting on it so it gives an agitating blast. I added a shut off to mine so I don't have to go to the spigot to turn it on and off. Beware if the heater core is on its way out it may cause it to leak. I have used mine for years and never had a problem though

[Kiesel]

I wound up just using the garden hose with a brass twist nozzle on it and a small ball valve behind that. One of those with the little black lever for on and off. Opened the nozzle so it was at full stream, closed the ball valve, stuck it in the heater hose and slowly opened the ball valve back up. Nothing happened at first, so I closed and went to the other hose. Slowly opened up and the water started coming out the first hose. Brown at first and then clear. Kept opening valve slowly until good water flow. Hookes everything back up and heater works great!