Temp gauge mystery

Add a few more. bad connection at the harness pin to the pc board trace, Clean, flux, with electronics compatible flux and re-solder. Corrosion at the gauge nuts-- loosen/ tighten them a few times

Try disconnecting the sender and solidly grounding the wire, then cycle the key and watch the gauge. It should immediately peg with key turned to run. Do not leave it powered for long. If so, sender. DO NOT overtighten sender. Could also be corrosion in the bulkhead connector for that wire.