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Has the thermostat on the left come apart? Hopefully there is not a brass cap stuck in the block/head.
This truck came from my father in-law who bought it new in 78 so it could be the original. Did they look like this back in the day?
New one is on the left. We ran out of day light before we could put the new one in, but now want to make sure we don't have to fish for a piece of brass somewhere.
The one on the left looks just fine and probably is more reliable than the new one you bought. The new Stant T stat on the right is junk, nothing but issues with them. Buy a Robert Shaw or Motorcraft 195.
If the old one is stuck open it should be replaced, especially with winter coming. If you're wondering about the performance of a thermostat you can heat it in a pot of water with a known good thermometer on your stove and watch it open while checking the temperature.
We did test the new one on the stove before we installed it and it opens properly. Topped off coolant yesterday and did a test drive. We can now see the temp needle rise, then fall once the stat opens.
