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Those holes are to allow coolant to circulate when the thermostat is closed, I think I drill two 1/8 holes in my thermostats also for that very same reason, let the air out so I can fill the cooling system up completely without having to wait for the thermostst to open and it lets a small amount of coolant to circulate while warming up




So more than likely they drilled a hole way to big and the car never warmed up and put a plug in it with a 1/8 hole in it. So the car would warm up.