In the process of restoring a 1963 Thunderbird convertible and I need some help isolating an overheating problem at highway speed. 

Status to date:

1-No apparent coolant leaks.

2-No heating issue at idle.

3-Heat sender seems to be operating fine.

4-New thermostat opens and pump is circulating the coolant.

5-Checked for head gasket leaks using block tester no combustion gases detected.

6-Radiator appears to be in good shape.

7- Radiator fan has excellent air movement.( There is no fan clutch)

8- No sign that water is getting in the oil.

9- Radiator hoses and belts recently installed.
Any ideas would be appreciated.

Thank you

A few ideas for you to perform. Use a hand-held infrared thermometer to check the temps at the upper and lower hoses. If you don't have one, this might be a good opportunity to purchase one. There should be an appreciable difference between the two measurements, say 180 at the top hose and 140 at the bottom hose. While you can only do this at idle, it does provide an indication of the health of the radiator. If you can take measurements across the radiator's surface and look for hot spots. A hot spot might indicate a blockage. An alternative is to simply yank the radiator and have it flushed preferably boiled out. The clue you provided is it happens at highway speed where the radiator can't cool enough at high speed, but is OK at idle.

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