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2008 GMC Sierra Heater blowing cold air

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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

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  • 4 weeks later...
  • 3 months later...



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...

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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

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  • 1 year later...

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!

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