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Your symptoms are unfortunately consistent with a blown head gasket. The water level is below the heater and the coolant temp sensor, so neither one works. You should be seeing steam at the tailpipe when the water is topped up, and bubbles in the radiator.


Park the truck on an incline, where the front of the truck is higher than the back and check for this. Bubbles = no bueno...

Otherwise, bad thermostat, not closing.

John B.