Question about GMC Jimmy

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No heat! changed fluid, thermostat. Was going to check control valve but don't know where it is. if that is not the prob should I change the core

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  • Mike Butler
    Mike Butler May 11, 2010

    Lets get year ,make & model, & engine for sure first. Did you have heat originally? Can you find your heater hoses where they go into firewall & feel them to see if both are hot?


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If you are getting cold air blown out of the vents than you know the system is working fine, you just need that warm air. So, since the steps you have already taken have not work you must change or flush out your heater core and see if a bunch of gunk comes out, if it does you can just put it back in if you want. Depending on what year your Jimmy is, getting to the core is alot of work so you may just want to replace it to prevent problems down the road.

Posted on Dec 31, 2008

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This problem is one i have encountered on a number of vehicle not just your brand. The problem seams to be in the controls switches and sending units. you have change lot of parts but did you check to see if you had flow through the heater core.Remove the heater hose's from the engine going to the fire wall. This is not precise in how you get the hoses off, however you do it? Once the hoes are off take a garden hose and try to run water throught the heater core. If you do not have a flow you need to find the closed valve. You may need to block off the hoses to the engine so you can run it for a vacume system. You may need to cut and put splices in the water hoses to do this. As a lot of engines you can not get to the end of the hoses.
This is not a simple fix as you would think. As i have found the heater hoses are getting warm or hot with the engine running but it is still cold inside the car. The word is no water flow no heat and if you have flow you have a vent door stuck closed. Air conditioner controls can have effect also. you may also may need to check the water pump out put to the heater core. This needs to be done with all the anitfreeze removed so as not to lose it on the ground. If you do remove the anitfreeze you can check the heater core flow by only removing 1 (one ) hose for the firewall to the heater. if you have hose off remember to put the garden hose in the radiator with it one as not to over heat the engine while it is running. you had heat before it a matter of tracking down where the flow has stop or the cause like a bad switch, broken wire or vacume hose.
good luck

Posted on Dec 31, 2008

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SOURCE: i have a 94 lexus

check the temp of the heater hoses going into and out of the heater core. they should be the same temp. if not then the heater core may be clogged up.

Posted on Oct 31, 2010

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98 ford ranger no heat have changed thermostat water pump heater control valve

Maybe the heater core is plugged or you haven't bled it properly?
If the heater hose returning from the core to the water pump is NOT hot, while the hose going to the heater core from the thermostat area IS hot, then the heater core is plugged probably.
You need to remove the heater hose while cool,from the thermostat area and remove the hose from the water pump area (that's the outlet of the flow thru heater core) AND THEN take a garden hose with p-is-tol grip type sprayer,and blow water back thru the outlet hose out the inlet hose. This will blow any bad plugging material out the way it got in.
Don't apply too much pressure if it is plugged bad. The heater core isn't made to stand 50lbs. water pressure. don-ohio (:^)

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your heater switch is not working.take it to shop and they will show u were it is. unless your thermastat is stuck, does your engine heat up? if so change your thermostat. other check you switch..mudman1966

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Heater blows cold air no heat

There are three thing that can go wrong here. I will give you the two simple ones first. first, check to make sure the thermostat isn't sticking or stuck. if it is, change it. two, Hot coolant going from the engine to the heater core, is controlled by a valve at the engine(these do go bad). Check the line when the engine is warmed up by turning on the heater(I know), and feel the line after about 5 minutes. The line should get hot, or very warm. The third is your heater core is clogged and needs to be replaced. hope this helps you

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2002, VW Passat, 2.8 Liter, 30V, ATQ No heat, Climatronic System. Control module is good, Fluid levels are good, thermostat good. Can't find the Heater Control Valve, need the location for this valve???

No heater valve on this car. Heat is controlled with temperature doors that blend the ac and heat to give you the desired temp. The core is probably clogged. It usually does not clog until the coolant wont pass through it but the fins get cruddy and don't transfer heat to you. Try taking the hoses off the core and blast water and air through it then retry the heat.

Nov 18, 2010 | 2002 Volkswagen Passat

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I have a 94 lexus that is blowing no heat.I have changed the thermostat,removed the control valve,changed the raditor fluid,bleeded the cooling system.But still have no heat.checked the heater core by...

check the temp of the heater hoses going into and out of the heater core. they should be the same temp. if not then the heater core may be clogged up.

Oct 31, 2010 | Lexus LS 400 Cars & Trucks

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I have a 93 delta 88. heat comes and goes. we have changed the thermostat, and checked fluids. the car does not run hot. but the heat will work one day and blow cold the next.

Check the vacuum diaphragm in the heater hose. It's a little metal valve with a hose going to it. When the heat is turned on the vacuum line opens the valve letting hot water pass through the heater core.

Mar 14, 2010 | 1988 Oldsmobile Delta 88

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I have no heat. We have changed the thermostat

The valve that moves to let the water into the core could be stuck. also check the under dash vac. hoses that control it.

Dec 11, 2009 | 1998 Pontiac Bonneville

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Blend door functions, why no heat? exactly where is heat core?

First, there are two doors inside the heater. one door changes air flow between heat and A/c, the other changes where air is going (vent, floor, defrost etc). You did not mention if when you moved the door by hand, if you got heat when you did that, or, if the motor you removed had vacuum going to it and if it was functioning when you moved the dash control. Next, you don't need to cut holes in the box to check the core. If the hoses going to and from the core are getting hot, likely you have flow going through it. You can do a temporary visual check by replacing one hose with a clear plastic one and observing flow. (don't leave plastic in place as it will get soft when hot and will eventually fail.). You either have a vacuum problem not allowing blend doors to move, a second door that is jammed, an obstruction, internal or external (leaves, mouse nest etc) of the heater core.
If yours has a heater control valve that is installed on the heater hoses, check that as well (failure default is usually full on which will normally give you heat, but they can stick in the wrong position as well).
Last, I know removing the box is a real pain. Generally though, in the end you will need to remove it if the core needs changing. I'd first attempt to flush the core, independently of the system by removing both hoses at the engine and draining the core, then filling it with flush, waiting a half hour or so and then blowing it out both ways with a garden hose.

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No heat,ch.thermostat,water is circulating,blower goin strong .

Check the 'heater control valve'. This valve opens when u turn on the heat switch on the dash, so the hot coolant will flow thru the heater core and provide heat. So although the coolant is circulating in the engine, the valve controlling the flow to the heater core is stuck closed, or not working.

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