Slow fan clutch failure?

[str12-340]

so, 1970 Dart, mildly hot rodded 318. New Cold Case radiator, new water pump, new high flow 160 thermostat. Stock fan and shroud. Car runs cool until you have to sit in traffic, then temp slowly creeps up to about 200. Immediately cools down once you start moving moderately - even 25 mph brings it back to 160 in a couple minutes. Is my fan clutch slowly failing and not spinning the fan enough at low or no speed? Is there a way to test it on the car?

 

[Desoto291hemi]

Sounds like the classic symptom of a bad fan clutch
No test that I know of
Just replace it and the temperature will go back to normal

Tommy

 

[dano]

I have the same set up with a 340, 180 thermostat and fixed fan, same issue. Tried to tune it out to no avail. I know modern cars run hotter, 200-210 is OK for them. Do you get boil over?

 

[Darter6]

To check you clutch fan at operating temp have someone turn the key off while you watch the fan.
IF it spins more than 1 rotation after the engine has stopped it's getting weak.
Another is after the engine is stopped check by hand. You should have resistance and not spin more than 1/4 rotation when you let it go to none at all. Just a thought..