1999 GMC Sierra Logo
Jay Anderson Posted on Dec 27, 2012
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Heater blows only cold air

Heater only blows cold air even tho water circulates through core

2 Answers

Marvin

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  • GMC Master 85,242 Answers
  • Posted on Dec 27, 2012
Marvin
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Air Temperature Control Blend Door Failure No Heat or no AC (dependes on where door sticks)

In all modern cars there is a tiny DC electric motor driven gear drive that moves a plastic door that blends heated and cooled air from the AC and the heater core, this is called (interestedly enough) the air temperature control blend door actuator. The motor that moves this door fails because the door starts sticking from warping, most common complaint is no heat but it can be no AC as well. To replace these parts (blend door and blend door actuator) you must remove the entire dash from the car. Always have this problem confirmed by a dealer or qualified repair shop as it is a very expensive repair.

Other causes of no heat in the cabin are engine thermostats that are stuck open (engine runs cold) or air bubbles in the cooling system.

Mike

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  • Contributor 55 Answers
  • Posted on Nov 16, 2013
Mike
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It is true that blend doors and motors are a bear to change out. It is not always necessary to remove the dash though. and on older cars and trucks that may not be cost efficient you can jury rig doors in positions that will give you heat or cold when you want it.

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5 Related Answers

emissionwiz

Marvin

  • 85242 Answers
  • Posted on Mar 10, 2009

SOURCE: my air blows hot even tho my climate control is on cold

U have a stuck air temp control blend door, either the actuator motor is defective or the door is warped and has jammed, one thing is that u must remove the dash to rcheck replace these parts.

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Anonymous

  • 2920 Answers
  • Posted on Jan 26, 2009

SOURCE: 2002 GMC Yukon blows cold air from the front, rear works fine

check you antifreeze level and make sure its full

Anonymous

  • 144 Answers
  • Posted on Sep 04, 2009

SOURCE: the rear a/c on my 1996 gmc suburban is blowing

the problem is in the rear expansion valve. this valve is located at the rear evaporator core. i am assuming the freon level has been checked and is ok. this is a common problem with these vehicles. hope this helps!!

Anonymous

  • 1 Answer
  • Posted on Oct 09, 2009

SOURCE: My 2000 GMC Jimmy has been blowing cold air. We

airjoc does not have a clue to your problem, I take you want to have warmer air not cooler ac. check the control system, if you have a vacum system tou may have a leak, if it is controlled by voltage, check the control panel and the control box. Also open the hood, place the heater on high but not on any defrost setting preferrably on just the floor setting. Start the engine and see if the ac compressor is running. If it is, the control panel is bad.

Anonymous

  • 173 Answers
  • Posted on Dec 07, 2009

SOURCE: Heat not working on 98 gmc jimmy. Replaced

Flushing the heater core is fairly easy just remove the 2 hoses going to the heater core and use a different hose if you have to and use a garden hose to backflush the blockage out..this may take as much as 5-6 times also check for vacumn lines missing or disconnected or breaks pinched ect... some where in the vicinity of the passenger side under hood by the a/c lines ect. good luck hope this helps

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Why is my heater blowing cold

Most likely it is the heater control valve that shifts air from blowing across your heater core to your air conditioner core.

If your fan works and your car is fairly new..meaning your heater core isn't dirty and full of craap...then it is the climate cotrol valve.
If you are lucky, it might be a thermostat that is bad and blocking hot water from entering your heater core. Bottom line, if hot water doesn't circulate thru that little heater core, you won't get heat.
You might be able to look under the hood and grab the hot water lines that go thru the firewall and feed your heater core. If they feel cold to the touch after engine has warmed up, then you aren't getting circulating water to the heater core. If they feel nice and warm, then the heater core is fine, but your climate control valve has closed off the fan that blows across the core to disperse heat into the cabin
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Heater core could be blocked up, or the blend door is stuck on the cold side of the a/c evaporator or the blend door is stuck in the cold position.
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If you look by the firewall there are two hoses. They go into the heater core. When the engine is warmed up, and the heater on. Feel if both hose are equally warm. Be carefull you don't burn yourself. A valve lets the hot water circulate through the heater core therefore providing heat. Also feel if the heater core is warm under you dash on the passenger side. If it feels cold then water is not circulating. Locate the valve, which is usually vacuum operated. See if there is vacuum there when the heater is in the on position.
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That is usually the first symptom of low coolant. Not enough water in the system to circulate through the heater core. Has it been running hot?

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The heater in your car depends upon a flow of hot coolant going through the heater core in order to get hot air into the car. If the heater core is partially plugged, it will restrict the flow. If you have a head gasket that is putting exhaust into the cooling system, it will eventually get into the heater core. When this happens there is no coolant in the heater core to make the air warm. If the water pump vanes have corroded away the result will be very poor coolant circulation causing the heater to not blow hot air. If your car is equipped with a heater control valve that is not functioning properly, the coolant flow will be restricted. One or a combination of these factors is likely the culprit in your case.
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The heater in your car depends upon a flow of hot coolant going through the heater core in order to get hot air into the car. If the heater core is partially plugged, it will restrict the flow. If you have a head gasket that is putting exhaust into the cooling system, it will eventually get into the heater core. When this happens there is no coolant in the heater core to make the air warm. If the water pump vanes have corroded away the result will be very poor coolant circulation causing the heater to not blow hot air. If your car is equipped with a heater control valve that is not functioning properly, the coolant flow will be restricted. One or a combination of these factors is likely the culprit in your case.

if the car is not overheating, the thermostat is working fine.... replace or flush the heater core multiple times to try and eliminate sediment build-up.
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