Question about 1990 GMC Sierra

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My heater don't get hot, my heater core is good and circulates well, i think possibly the heater door don't work,maybe the mtor is bad on the actuator

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Well 1st ensure the hoses to the heater core are operating by touching them 1st the top hose and then the bottom hose they should both be hot to the touch if only the top hose is hot and the bottom hose is cold most likely your heater core is plugged up as this occurs from not using it and sediment sets in and connect a garden hose to the top and the bottom hose at the base and the top hose "Do not "use full force" when doing this as you risk rupture to the core ...when flushing it
this will help dislodge sediment and you should see gunk come from it ...
if both are hot to the touch then it may be the cable inside under the dash has slipped or a vacuum hose is pinched or come unconnected ...visual inspection on both issues is the only way to ensure they are working properly .
consider as well antifreeze levels and possible stuck thermostat ..{ just a note }
as well make sure the vacuum lines are especially the one under the hood are all connected as this is Vacuum that works the system ,...check for dry-rot and possible disconnected ..replace as needed
inspection of the vacuum hoses to the cluster under the dash should be fairly easy on your vehicle and use a flashlight drop light and even a hand held mirror in determining the exact cause to begin make sure the vehicles is running while working the levers and the switch this is where i would start 1st

Posted on Dec 12, 2009


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Nothing in or around the heater core could make a noise. The core is just a small radiator with coolant circulating through it. And if you feel both heater hoses, and both are hot on a warmed up engine, then coolant is circulating through the core and the heat should be working. There must be a door in the heater case that is not letting heat come out.
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saailer_214.jpgMOTORCRAFT Part # YG229...$20

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Follow these steps:

Check coolant level, then start engine until it is warm and the thermostat opens. You will tell when it does when the needle on your temp gauge goes cold when it reaches around 180-200* and then goes back up. Now turn on your heater full blast and go to the engine compartment. You will see a heater valve located on the passenger side, next to the washer reservoir. It is a black thing with 4 heater hoses connected to it. Feel the hoses. When everything is working correctly it should be like this:

Hose going to waterpump will be hot
Hose going to (I think the thermostat housing) will be hot
2 Hoses going to heater core (mounted behind firewall) will be hot

1. If only 1 heater core hose is hot, heater core is clogged--
UNCLOG: remove 2 hoses going to heater core when engine is cool, hook water hose end to one of the sides and flush out gunk from heater core. Reconnect hose and you should be good.

2. If neither hose core hose is hot, heater control valve is bad or vacuum is bad--
Test Vacuum: remove vacuum line from heater control valve, you should feel the vacuum **** on it when pressed to your cheek and the heater control valve actuator should move when you unhook the vacuum line. If you have a good vacuum, it's probable the valve available at autozone part # 5941 for around $21.

To get heat while you wait to replace part, unhook silver arm on heater control valve that is hooked to the plunger. This will give you heat all of the time, but it is controlled by the bottom hot/cold switch on your heater control unit so you won't get hot in the cab. It will just circulate heat through the core.

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