First I'd like to say that I've been creeping this forum for about 5 years now and you guys have been very helpful in dealing with my POS car (05 Grand Prix GT). I'm still a big noob when it comes to cars though. Anyways I have ran into a problem that I couldn't find using the search bar. My car has been leaking coolant slowly for like 4 or 5 months now but being a college student I haven't been able to figure out where or fix it due to lack of money or knowledge. Then yesterday as I was driving down the interstate at about 75 mph and the temp gauge went crazy going up and down randomly and then overheated. This was all within 15 minutes of me starting this journey. I thought I was probably low on coolant but I checked the overflow tank and all of the coolant had been pushed into the tank making it way too full. I let my car sit for a couple of hours and the coolant all stayed in the overflow tank. I was able to have a buddy of mine load me onto his trailer and take my car back to my apartment. Where it is now sitting because I'm afraid to drive it and risk damage to the car without knowing what it is. I've seen similar posts about it being the thermostat or a blown head gasket.. But none of the posts mentioned the coolant being pushed into the overflow tank. My car has 140,000 hard earned miles on it. And yes in my younger days I put green coolant in my dexcool... So that's part of my problem there I'm pretty sure of that. Any insight would be awesome as to what may be wrong (other than I own a Grand Prix) and what I should do about fixing it would be much appreciated.
