Yesterday my 88 was idling hot , 240 ish is all I ever let it get to , I threw in about half a gallon because that's all I had . Ran it today and on the way to get it inspected the t - stat got stuck and the temp shot up maxing the gauge. I took it out and got the car inspected without it. I went straight to Napa and got a replacement t - stat and some more coolant. It took about another half by the engine and I ran it a bit. Noticed the overflow tank was low so I filled it to the middle between the full and add. Started the car again and it sucked the coolant down to the add line. I know I didn't fill it or burp it properly could that make it suck up the coolant? Can't find any leaks. Filled the car at the engine when I got it 2 weeks ago but didn't notice the overflow.

There should be evidence of fluid loss if you 'maxed' the gauge. Maybe the overflow tank overflowed...or something. You would have air in the system after that, and you are doing all the right things to fix that. It's possible that you have a puncture in a hose, which releases coolant only when everything is hot and under pressure..and leaves no trace.

quote Originally posted by Gall757: There should be evidence of fluid loss if you 'maxed' the gauge. Maybe the overflow tank overflowed...or something. You would have air in the system after that, and you are doing all the right things to fix that. It's possible that you have a puncture in a hose, which releases coolant only when everything is hot and under pressure..and leaves no trace.

Maxed gauge was the temp gauge due to the thermostat not opening and letting coolant to the radiator.