Q: Evaporating coolant

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Been running out of coolant faster then usual so I tried to burp the coolant system to get rid of any air. Long story short at 190-200 temp the coolant in the reservoir wasn't moving anywhere except steam was coming out of the coolant fill. I've already done the cardboard under the car so no leaks and I checked the oil and it's not milky. After a long drive I'm noticing the coolant overfill has steam coming from it. Best guess is it's either the pump or the thermostat valve that's not getting the coolant through out the system. If someone could give me a little advice that would sure be great. Thanks for listening

My car has 185204 miles.
My car has an automatic transmission.

There are two things you might want to ask yourself. First, are you using a 50/50 percent ratio of coolant to distilled water? If not, this could cause the coolant to evaporate when it gets above 170 degrees. This is common when you’re using more water than coolant. If the coolant is more than water, it will expand and push through the coolant overflow reservoir since coolant expands as it heats. I would recommend flushing your coolant system fully; and check with Pontiac about the procedure for priming the coolant system correctly. This will eliminate air bubble issues.

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