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My 1996 GMC Sonomas interior heat has inexplicibly stopped producing hot air. It used to work fantastic. My mechanic thinks that the heater core is clogged. He says to replace this would cost around $600. He believes adding an additive might work. What do you think?

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Your mechanic is likely correct. Pretty much any vehicle will eventually develop a clogged heater core, especially if your antifreeze/coolant system has never been "flushed out" to remove lime scale. The antifreeze/coolant flows through the heater core and warms air that passes through it, but your mechanic has something similar to CLR that will dissolve what clogged it. This is usually still referred to as "boiling out" your heater core, but it's been decades since it actually involved boiling the thing to dissolve the lime scale clogging it. The products you can buy today that will "flush out" the engine coolant system are designed to dissolve and remove these clogging bits of crud, though if it's already clogged up it may take some time running the engine to get through to the heater core to clean it out. Most mechanics will simply recommend replacing the heater core, because this is a much faster and much more guaranteed cure to the problem - even though you pay more for it.

Posted on Nov 14, 2010

  • Kevin Omel
    Kevin Omel Nov 28, 2010

    Additional note: This is why the owners manual recommends using only distilled water in your engine's coolant system. Using tap water can leave behind lime and calcium deposits, and these are what clog up things like the heater core.

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Heater hose diagrams


The interior cab heating system is identical to 4-wheeler cars. Two hoses connect to the cab heater core box, which is part of the engine radiator coolant system. If you're not getting heat when you turn on the heater fan, then one of the hoses is blocked or the heater core box is blocked up. One hose is inlet, which brings hot coolant from the engine to the cab heater core box. The 2nd hose is outlet which returns the coolant fluid from the heater core box back to the engine coolant system. Its likely these hoses and the heater core box are 20 years old and had never been replaced since the semi truck rolled off the assembly line in 1994. To diagnose for blockage, disconnect both hoses from the engine side, then connect an air blower to one hose to push pressurized air into this hose. Air should be coming out the other hose. If there's no air, or less air pressure, then there's blockage which is restricting hot coolant from passing through, resulting in little or no hot air when you turn on the "hot" air fan. Replace the heater core box first. If you're not getting hot air, then replace both hoses next.
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My 99 GMC sonoma with a 2.2L is not warming up properly


Possibly plugged or inerporable heater core. Possibly heater core valve (this diverts hot air through your heater system) When you flushed the block, was there a lot of contaminant? 95% of the time, cold heat means thermostat. Also there are a lot of really cheap after market thermostats, you can buy one as low as $6.00. That being said, just because a thermo is new doesn't necessarily mean it opens/shuts when it should. You can look on You Tube on how to test one in your kitchen with boiling water,.

So three basic things affect interior heat temps. Thermostat, plugged heater core/.valve, and insufficient coolant or liquid to tranfser heat. If your interior coolant (engine temp) is working normally and your vehicle reaches operating tempature according to the needle, your thermosat is probably not the cause.

My bet is somehow, your heater core is plugged so that their is not enough "flow"

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If the heater hoses are getting hot, and the radiator is full of coolant, you could have a problem with one of the blend doors under the dash. The blend doors direct air flow thru the heater core and the vents.

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If it won't produce heat then you have an issue with your heater core. The heater core could be bypassed, in which case no hot coolant will get to it to produce heat inside the cabin. The heater core could be stopped up with dirt/debris, in which case the heater core can't radiate heat to produce heat in the cabin. The ducting in the dash could not be allowing the hot air to flow to the vents - therefore no heat in the cabin. I'd start with the heater core and check to see if it is bypassed. This generally happens on older vehicles because the parts can be harder to find.

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I have an 1996 Nissan Sentra GXE i live in ND and the heater only blows luke warm air while at full operating temperature. I replaced the thermostat and flushed the heater core which seemed clean....


Sounds like it's the air control flap which determines the mix of heated and fresh air. This could be vacuum controlled or manually with a cable.

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1 Answer

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2 Answers

The heater only blows cold air,the radiator is full,thermastat is working and the engine temp is within normal ranges.


I've had this problem. 2000 GMC uses a closed coolant system. Meaning takes and returns coolant from and to the Reserve Bottle and has little air in the engine and heater systems.
If everything appears ok but no heat. "Air Lock" is the most likely cause. (With interior heater set to full heat) Solution; 1. Make sure water pump works. 2. Drain about 1/2 engine coolant into container. 3 Remove both heater hoses from engine side and flush heater core in both directs through heater hoses with garden hose. 4. Reattach heater hoses in correct locations. 4. Refill engine with coolant. 5 Start engine and run for 2 min., use caution when topping up coolant.
6. Most 2000 GMC have twist off caps with plugs located mid way on heater hoses. with engine cold, start engine these caps may be loosened to allow trapped air to escape. Do not attempt with engine hot due to hot coolant injuries. This may take more than one attempt to remove all air from heat system. 7. Lastly most 2000 GMC water pumps have a bleeder plug,( 3/8" square driver plug) located on the water pump. With egine cold this plug should be removed to remove any air. Reinstall plug with thread sealant.
Hope this helps cheers

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