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Adding to Strange Mud: If heat works when you accelerate, it indicates hot water only flows through core during hi RPMs. Possible causes are low coolant, bad water pump, under driven water pump.
Also, I may be off base here, but I have heard that the Coyote has some additional coolant gates beyond just the normal thermostat. You may look under the hood with the engine warm and touch hoses, following the heat. Get to a junction and the next section is cool, there might be valve not opening.
 
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