Heater blew nothing but cold air that evening

Tiny
MIKIE
  • MEMBER
  • FORD F-150
I replaced the stat in my '97 Ford F150 - 4.6L. The heater worked perfectly the next morning. It blew nothing but cold air that evening and always since. Other than possibly having the wrong stat or a defective one is there any other reason I would have this problem not withstanding a clogged heater core (which it's not). By the way I tried to donate to your cause but the link didn't work (some kind of error).
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Monday, February 5th, 2007 AT 7:05 PM

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Tiny
MIKE H R
  • MECHANIC
  • 3,094 POSTS
Try starting the engine when its cold. Holding the hose closest to the thermastat. The hose will stay cold until the thermastat opens. You should feel a sudden temperature change. Means the thermastat working properly. If the temp rises slowely than may be a stuck open thermastat. Have seen new ones that were bad. If the thermaste works ok then the heater core might be vapor locked or clogged.
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Monday, May 10th, 2021 AT 10:36 AM

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