after filling the coolant system fully, and driving approx 30 miles, much of the coolant has boiled out.fan clutch recently replaced. a/c fan kicking on properly. at startup, thermostat opens at 170-172 degrees. coolant appears to be flowing properly. possible radiator plug, or water pump?
Make, model, year, miles, etc would be helpful. Has the radiator cap been replaced? A cap that does not hold pressure would cause the coolant to boil over. Otherwise a leaking head gasket perhaps?
Autozone will loan you a cooling system pressure gage. The gauge is VERY simple to use, when the engine is cold remove the radiator cap & install the gauge in it’s place. Follow the directions that come with the gauge & I hope you dont see a bouncing needle.
As edb1961 mentioned the radiator cap might be shot. The gauge will also test for a bad cap.
Keep us posted on your test results.
The radiator cap is definitely the best place to start. If the cap proves to be okay, and if the cooling system holds pressure, then the next place to look should be the water pump. The pump might be just plain worn out–but if your vehicle is relatively new, this is not likely. How about sharing some details regarding the make, model, model year, and odometer mileage?