I need some help. What test should i run to see if i have a bad thermostat or a blown head gasket? Ive had the needle get close to the red line but never touched. Went take a ride and the needle would get warm and warmer them go back to normal. It did this all the way back home (65 miles). I checked the oil its not milky and no white smoke coming from the tail pipe. I was low on coolant and when I put more in about a month ago it was good till today's ride. I was hoping that I had a crack in the overflow tank that I could fix. But it dont look like it. I had a appointment with my mechanic next week to adjust the TBS because it wasn't running right. Could this be connected to a blown end gasket?