Defective thermostat?

[TedC]

I decided it was time to change the coolant and thermostat. After reading several posts, decided to try out a 180 degree thermostat. Even Gordon Killebrew was recommending 180 over the stock 195 in the L98s. The first thing I noticed was faster warm up, and once coolant temp hit 185 degrees, it dropped quickly to 176. Driving on the open road, the engine oil temp maintained a rock solid 185, and the coolant a rock solid 176. Even after pushing the car hard, both would settle back down to the same readings. Sitting still for a few minutes, both temps would rise until the fan came on when coolant temp peaked at 233. Back on the open road, readings dropped back down to the same 185/176. No issues with cabin heat. When I tested the two thermostats, the 195 thermo started to open at 175 and was fully open at 210. The 180 thermo started at 180, and was fully open at 200. The 195 thermostat never provided rock solid temps like the 180 when driving on the open road, so I suspect it was going bad.

 

[WhalePirot]

I have replaced the stock Stant with a RobertShaw 180. It finally solved my overheating issues with the big motor, as the oriface is larger.

I learned that the thermostat is constantly opening and closing; keeping the coolant in the block to absorb heat, then in the radiator to eliminate it. It does not, as I had assumed find a partially open, modulated amount of opening. It's more like an on/off switch.

 

[TedC]

WhalePirot, Really impressed how consistant the dash coolant temp readings are now. I'm sure a 195 degree thermo would have produced similar results but at a higher starting point. Something else I'm noticing is that when I open the hood there is less heat coming from the engine compartment. The 15 degree delta in thermostat ratings does make a difference.