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no heat...again


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When I got the truck I replaced the vacume canister and got heat. Then there was a blockage in the radiator, changed it got heat. Now that it's getting chilly here....no heat again. All the hoses are hot except one of the hetaer core's. I fludhed water thrugh that, no blockage. It seems like the little door isn't opening and closing when I go from hot to cold. You can usually hear a little soft pop. No pop! Always cold. Besides broken or disconnected vacume lines (seem to all be good) is there anything else that will suddenly make the door stop moving? Winter's comming soon!

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Little door? The doors, or flappers, control air flow. If the hoses feeding the heater core are not hot, there's no coolant flowing through the heater core and that has nothing to do with doors. The flow valve is vacuum controlled, if you have a vacuum leak perhaps that's not working. Have you checked it?

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What year truck? On my 87 the doors controlling where the air goes (windshield/vents/floor) are vacuum controlled. But the diverter that forces the air either through the heater core or around it is mechanical using a steel cable (much like a parking brake cable). I disconnected the vacuum line from the valve in the engine compartment. It defaults to hot without any vacuum to it.

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