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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1 Answer

Heater core or thermostat


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.
The noise may be from the blower motor and its plastic fan assembly. It should be pretty easy to take out the blower motor and check for any obstruction.

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My Expedition doesn't transfer ac to heat. .blower works fine , no leaks..slight smell of antifreeze but goes away very quickly.no overheating just cold air blowing if as dial is set on heat/warm..Please...


LG, first you need to make sure coolant is circulating through the heater core. let the engine reach normal temperature, locate your heater hoses going to the heater core, they both will be near the middle of the firewall a little towards the passenger side of vehicle, be careful but touch both hoses and make sure they BOTH feel hot to the touch. If both do, then coolant is circulating through the core.

Next, you need to make sure the switch in the dash is working the blend door. The blend door is located right in front of the heater core, almost right behind the radio. If you take the little filler panel off at the bottom (depending on year model) you should see the blend door actuator, it should be white in color and has a set of wires (maybe 6-8) that all goes into 1 plug that plugs into it. With it hooked up and truck running as you turn the temp know from cold to hot you should be able to see the actuator turning the blend door rod.

It's not real easy to see, but if the actuator is turning and you have hot water/coolant circulating through your core, then you may have a broken blend door. The only way to fix it would be to go into the air box. Unless you have experience doing this type of work, I don't suggest you trying it. It is a complicated job/task to even get access to the blend door in the air box. Hope this helps you identify your problem.

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No heat ,


Check the vacuum lines to make sure they are correctly connected and in good shape. If they are not, then the system wont move the dampers to allow hot air into the heat vents. A second possibility is a clogged heater core, which is not allowing hot water to circulate through it. As you say the head has been replaced, I would have someone insure every thing was reconnected correctly. It could be the hoses to the heater core are reversed!

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I have a 2000 jeep wrangler when I turn on the heat or defrost I can hear it running but I get no heat. What could cause this heater coil, thermastat or something else maybe?


Is coolant level full? To check if the heater core is circulating coolant, feel both heater hoses where they enter the firewall-both should be hot when car is at normal operating temperature. If only one is hot, look for a heater valve in the heater line-make sure it is open so the heater core circulates. Not all vehicles have such a valve, in fact, I think most don't have the valve, but you could check. The valve may be cable operated or vacuum operated to open and close. If no valve, and one hose is cold, there may be an air block in the heater core. Crack a line loose to get coolant circulating.
If both hoses are hot, it is probably the doors in the heater case not operating properly. Watch the door actuators when you move heat from cold to hot, and when moving from defrost to another setting. The temperature blend door is probably on the passenger side of heater case. These doors may be cable operated, vacuum operated, or electrical motor operated. If you watch for their operation, you might see or hear the problem with them not opening or closing properly.
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If both heater hoses are hot

Jan 01, 2013 | Cars & Trucks

1 Answer

Heater is not blowing hot air


How old is the coolant in your car? If you haven't changed the coolant in a couple of years, you may have sediment build-up in the heater core that is restricting full circulation in the core and preventing good heat output. Feel both hoses at the firewall that go to the heater core. Are both equally hot when the engine is warmed up and heater controls on high? If only one is hot, and the other lukewarm, the core is partially plugged. Pull off both hoses there and run a garden hose at low volume backward through the core. If not sure what is backward flow just run the garden hose through both openings and see if you can remove any sediment or blockage.
If both hoses are hot, the core may be circulating fine, and the problem may be in the heater case's doors not opening fully- I think that would be the temperature blend door. When heater controls on full heat, the door should be fully open to let blower air pass across and through the heater core to pick up heat and pass on through to vents.

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Heater does not work. Blower works but no hot air


Check your heater core for good circulation. With engine running, heater on full hot, your inlet and outlet heater core coolant lines should both be hot. If one is much cooler, you may have a plugged, or partially plugged core (from corrosion, scale, rust, etc.), or an air pocket preventing circulation. Pull off the outlet line and see if a steady stream of coolant flows as it should. If you have good flow through your core, then it is possible a temperature door inside your heater case is not opening properly.

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Heater will not blow out hot air in my 1988 ford F250. Thermostat and hoses are all good. Is there a sensor that may be causing the problem and where is it located?


Hello! There isn't a sensor that would disallow heated water to circulate through the heater core...If the temperature gauge rises to normal...And the upper radiator hose is hot along with the lower hose ...I can conclude water is circulating through the block...Question then becomes; Is it circulating through the heater core? And the answer to that is, I don't think so...With the heater control turned on high feel the outlet heater core hose (facing engine it's the one on the left) if it feels cold or not as hot as the inlet then one of two items are at fault...#1...The Plenum assembly, which the heater core sets in has an open vacuum line...#2...The heat control valve cable or the valve itself is not opening...See picture of valve below...It is on the firewall...Let me know if heat is passing through the heater core...If hot water isn't passing through the core and the valve is opening than the core itself is blocked...send a comment...Guru...Saailer

saailer_214.jpgMOTORCRAFT Part # YG229...$20

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

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Mercury 2003 gand marquis heater does not blkow hot air the blower motor blows but cold air


One of 2 things could be happening. #1, the valve that allows coolant to circulate thru the heater core is not functioning. #2 the door that is actuated to allow cabin air to circulate around the heater core is not functioning properly. Possible causes, linkage to the valve, or actuator not working. Also a broken valve to the heater core. A vacuum leak, or faulty actuator would cause the door not to function. You can narrow down the problem somewhat by making sure that both heater hoses have hot coolant in them. You can do this by simply feeling the hoses. If they are both circulating hot coolant, then the problem is the misdirection of air flow. To repair this, the cause will have to be diagnosed, could be anything from your dash mounted controls to an issue with vacuum or the ductwork or the door or actuator. If not in the controls, the dash may have to come out. Good Luck, I hope this is helpful!

Nov 18, 2009 | Mercury Grand Marquis Cars & Trucks

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1998 jeep cherokeesport


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.




Oct 25, 2008 | 1998 Jeep Cherokee

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