how do you know if the thermal clutch isn't working properly. car runs 180-185 degrees around town but out on the highway at speeds over 55 mph temp goes to 205-210 degrees.
Check your coolant level. If its full up to the filler neck, you've seriously overfilled the radiator. I suspect this may be the case. Read on....
To TEST the thermal clutch, warm it up to about 200F and see how much the fan spins relative to the engine. You can use a tach on the engine, and mark one blade on the fan, and use a strobe light (timing light) to see how much the fan actually spins. An oscilloscope or a digital video of the fan might avail you in assessing its angular velocity. I think if less than 80% of the engine angular velocity (rpm) then the thermal clutch can be said to be defective, IFF at operating temperature. I advise you use the "heavy duty" Hayden 2747 clutch. The 2707 doesn't cut it.
This suggests mechanical issues well beyond your fan. Get a timing light onto that 383! If its not 12.5 degrees BTDC on piston #1, then make it so. Also, check your coolant PUMP. THERMOSTAT may be sticking. I changed mine this past week, and Tilly runs a little cooler, given that this Stant Superstat opens a little quicker than the previous one.
Now, how is your TRANSMISSION? IT may be dumping extra heat into the radiator at highway speed. If so, then you likely have issues there.
How much crap is in your cooling jacket? Have you flushed the coolant?
Having driven Mathilda, a '66 Newport w a 383 2 barrel carb, for over 3 years now, I've had ample time to learn the cooling system. Summer in Tucson, AZ punishes any machine, and I thank the Lord that since last May, I've not had the temperature exceed 210 Fahrenheit. That sole occurrence came from running a winter mixture of about 60% antifreeze and too much coolant. The FSM instructs us to fill the radiator to 1.25 inches below the bottom of the filling neck. I check that with a ruler every time I check the radiator. If you fill it FULL, as many neophyte Old Mopar drivers will, you will have effects VERY like what you've described.
Check your coolant level first.
