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Discussion Starter · #1 ·
My 79 Mercury Marquis (daily driver) has great heat for about approx 20 minutes of driving, then slowly gets cold until no heat. I changed the t-stat. I think I got all the air out. What now? Why in the HELL would it work, then run out of heat? By the way, telling me I need to change the heater core might make me have an episode, so please be gentle
.
 

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Sounds like you have a big air pocket and after running for awhile it ends up in the heater core.After running the engine for awhile try grabbing both heater hoses and see if they are both equally hot.
 

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I'm with Frdnut. Sounds like low coolant level to me. Also does the car have a heater hose shutoff valve. It may be faulty or connected wrong if vacuum operated
 

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Discussion Starter · #4 ·
Capt, where would this valve be? I think I went down this road before when i was trying to figure it out a while ago. I dont think I ever really found the damn thing !!
 

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Discussion Starter · #6 ·
Well, the air is pushing right from the vents that I switch them to, its just subarctic.
 

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HAY V8SHADOW
Did by chance you move any vacuum lines under the hood??
It almost sounds to me like there is a thermo vacuum tree under the hood and as the motor warms up its shutting off the vacuum to your heat door in your heater. If there is no air block in the system this is the only thing that makes sence to me...
Tim
 

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Generally the heater shut off valve would be inline on one of the heater hoses.
As for no vacuum to the heater box controls. That would cause the heater to blow air to the windshield ( regardless of temp!). This is designed to at least give you defrosting on the windshield which is a safety requirement( in this Province anyway!) Stu
 

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On 2006-03-07 10:08, CaptVirgilHilts wrote:
Generally the heater shut off valve would be inline on one of the heater hoses.
As for no vacuum to the heater box controls. That would cause the heater to blow air to the windshield ( regardless of temp!). This is designed to at least give you defrosting on the windshield which is a safety requirement( in this Province anyway!) Stu
Hay Stu
I thought the same thing about the valve. But most of the cars mid 70's and up do have vacuum controls depending on if the car is high end or not.. (I have a 76 cutlass that has all vacuum controls). But your are right if there was a vacuum leak it would close most of the doors on the system execpt the defrost...
So I would say its most likely has a air block in it or a pluged core or hose...

So Shadow
Did you get if fuggered out yet?? and hows your back now??

Tim
 

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Discussion Starter · #10 ·
Im going to burp the air real good this time. I watch it real close and be patient to see all the air escape before I am satisfied. I am bearing with the back ache when I do work on the car, but with all the answers I got, I'm not so concerned that it is a serious problem, just needs the remedies advised to me! Thanx
 

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Hay Stu
I thought the same thing about the valve. But most of the cars mid 70's and up do have vacuum controls depending on if the car is high end or not.. (I have a 76 cutlass that has all vacuum controls). But your are right if there was a vacuum leak it would close most of the doors on the system execpt the defrost...
So I would say its most likely has a air block in it or a pluged core or hose...

So Shadow
Did you get if fuggered out yet?? and hows your back now??

Tim


I hear ya Tim. Sorry, I was talking about the two problems separately. Your correct though a lot of cars by mid to late 70's had vacuum controlled heater hose shutoffs rather than the old cable jobs. I seem to recall a lot of Olds had an integrated heater hose valve that screwed into the manifold and was used as the outlet nipple. I seem to remember replacing a corroded one in my garage working days(early80's)!
Stu
 

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Discussion Starter · #12 ·
I am running in with the heater going, filled it up to the top with fluid, and was watching the tiny bubbles come out. It kept spitting fluid out little by little so i closed the cap and put enough in the overflow reservior. Hoses are hot as hell and cold air just keeps blowing out. Its running out there now, and im just letting it do its thing. I dont know.
 

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HAY KEN
If both hoses are hot then there water going through the heater core.. It must be that the heat door isn't opening.
ti,
 

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I agree, I'm thinkin the hot/cold air door must be vacuum operated and is movig to the 'cold' position after the engine warms up for some reason. I'd find out for sure if the hot/cold door is cable or vacuum operated. If vacuum, find out which vacuum motor it is and start testing on that with a vacuum pump.
 

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Discussion Starter · #16 ·
I'm pretty sure everything that way is vacuum operated. Hell, I even see air tubing hanging down from time to time. I will have to see just where this "door" is, then see if I can watch it operate somehow. Besides the fact that this car is just BAD on gas, this heater thing is just about it's only "fluke". Thanks guys.
 
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