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Q: heater won't work while truck is idling on 2004 Ford F-350 Super Duty

The heater will not work when the truck is idling. I have replaced the water pump and flushed the radaitor. There was a bad antifreeze resavior tank which sense has been replaced. Once the truck doubles its idling speed the heater will work. however if the truck sits and idles the heater will stop puting out hot air. I am not using or losing any antifreeze from the resavior.
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how is your thermostat? I have seen thermostat valves break and cause very unusual flow. did you check to make sure that both hoses are getting up to proper temp, I mean they burn the heck out of your hands when you try to hold them, that is the upper and lower radiator hoses. How clean is your coolant? dirty coolant can plug the heater core, so it only flows with a lot of pressure, i.e. higher rpm. How about the heater control valve? does it work properly? Do you have auto climate control? If so, scan the computer, there may be a vacuum motor that is only working with more more vacuum from faster turning vacuum pump. How well does your vacuum pump work? these are some of the things I would look at, hope it spurs some ideas. also go here for the tech info on your truck's climate control and heater system
Best of Luck!
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