I have a quick question. I have a hummer h2. I added coolant to the reserve tank and the next day the temp gauge was at 200 and then it dropped all the way down to 160 and after a couple of minutes it comes back up. What is going on? Any help is appreciated.

 

Welcome to HF. Moved your question to the H2 section where questions about H2s go.

Did you bleed out any air in the system where the tank was added, or run it to temp with the radiator cap off so it could burp itself?

I'm guessing air? Check the coolant fluid level inside the radiator and reservoir.

Better guesses, please chime in.