Question about 1998 GMC Sierra

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Heat works sometimes but when it does its not hot.

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If the radiator is full of coolant, and the heater hoses are getting hot, the problem could be a blend door under the dash or the controller.

Posted on Jan 22, 2013

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the heater core may have gone bad or is cloged. if you can find the return line of the core pull off the hose and see if any thing comes out

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What was your temp gauge reading on the vehicle when this occured?
It could be a small plastic blend door in the dash that is operated by a small motor that when you turn your switch to hot the door moves to allow the heat from your heater core to be blown through ducts into your cab. Sometimes these can be calibrated sometimes he plastic door brakes. BUT i would start by bringing the vehicle up to operating temp first and checking it then.

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Feel the hoses going to heater core, one should be hot and the other warm. If one is cold or both are then heater core is clogged. Make sure heater control valve works and as vacuum to it and or the cable is free.

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Jan 10, 2011 | 2007 Ford F-250 Super Duty Lariat

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Since your blower is working and it is a heat issue,thats where we'll start,First thing is have you checked the coolant level in the radiator when engine is cold,make sure it is filled to top,next verify you have coolant in reserve ,Start your van and get it up to normal operation temperature,If your heat gauge on the dash works correctly it should be 190 degrees,Now lift the hood and locate the two heater hoses going into the heater assembly at the fire wall,Both hoses should be hot to hold on to,if both are hot and no heat inside your problem will be the blend air door in heater assembly.
if neither hose is that hot you need to replace the thermostat in the engine to get it up to 195 degrees.
if one hose is hot and one is luke warm then the heater core is blocked and to blame. hope this helps you.

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could be a heat door in the heater box, or depending on your heater control unit you might try replacing it.
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When you say the heat doesn't work are you meaning the fan doesn't blow out hot air or just doesn't blow out any air? if it blows cold air sometimes when you have the heat on then it could be poor circulation through your radiator or a weak water pump. make sure your radiator is clean on the inside, not too corroded or nasty inside. You can buy a radiator flush thing from wal-mart or any auto parts store. See if it helps.

Mar 01, 2009 | 1998 Chevrolet S-10 Pickup

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Heater blows cold air


With the car warmed up,feel the heater hoses,are they both warm/hot near the firewall in the engine compartment?If they are both warm/hot,then the heater core should have circulation through it and it may be a "blend door" issue in the heater a/c assembly.If they are not warm/hot then the heater core could be dirty and plugged up and needs to be flushed.
If the above does not help,a drain and vacuum of the cooling system and vacuum refill will remove any trapped air.
Sometimes,clamping off the top rad hose and holding the engine rpm at 2000 rpm will force trapped air out of the heater core.(Watch temp gauge when trying this)
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after checking the heater core hoses they were both hot I went in to look at the blend door and noticed it wasn't moving on hot or cold so I simply disconnected the lincage and moved to hot and its working ok. but its not completely repaired

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