[Michael C Henry]

That is a thermostat bypass hose. Replace it with a length of good quality hose preferably sillicone. Make sure the new hose is as long as possible to fit in the space , the metal tube ends are clean and smooth. use good hose clamps of the correct size, Do not over tighten the clamps , you can allways tighten them more later if necessary.
My brotherinlaw once had an 84 F250 that had problems from the start. Turned out that hose had a blister between the layers that affected the inside only and wasn't visable from the outside. The blister would shrink after cooling but when taken off immediately after running, was apparent and the blister had the flow blocked. Replacing that piece of hose fixed that problem.