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Have noticed temp gauge hardly ever moving off full COLD mark. Seemed to always take a while to warm up, and have decent heat out of vents...wont run you out of the truck but is "ok"...so I'm thinking thermostat might not be closing.
So I swaps out T-stat (195 degree), re-fill radiator, fire it up, set heater to high, and wait for full warm up....temp gauge begins moving.....gets to halfway, then eases down to about 1/4. I go out to run errands.....gauge runs up 3/4....drops down.....goes up, then drops down...it's all over the place. I'm stumped and turn to the collective knowledge here-in for guidance/ideas. Thanks in advance.
still sounds like bad thermostat. I assume you have all the air out of system ? Park with the nose uphill a foot higher than back end. That will make air go toward the radiator and not trapped on top. I cut a high heater hose on my F150 and installed a plastic flushing TEE.. that helps to blead out air when filling the antifreeze. An air pocket in the system can give up- down readings like you suggest.