I have your basic aftermarket three gauge setup; coolant temp, volts & oil pressure. This morning (ambient 40 °F) I started my '72 455, warmed it up for a few minutes (to about 120 °F) and proceeded to idle through the neighborhood. After about two minutes of this the coolant temp needle shot up to 200 °F and rising. I actually saw it rise fast (never saw that before). I immediately turned around, thinking the radiator was bone dry and headed home idling. Once I got home the temp needle dropped to 150 °F (again quickly). Opened the hood, the hoses were hot, no leaks, etc. I had no choice but to drive to my destination, so I watched the temp needle the whole way. It was a 2o minute drive and the gauge never went over 160 °F. Had the heat on the whole way. A few hours later, on the trip back home, the exact same thing happened. BTW, I have a resistive idiot light that works (and lights up early), but never fully lit at any time when this happened. Any ideas; aside from just changing gauges... Thank you!
Thermostat might be sticking and then "popping" full open once it got past "full open temp". 'stats have "dwell" as they begin to open at "listed" opening temp, but are not fully open until 10-20 degrees over that. I had one do that back in the 70's, used to warm the car up to about 160* and one time it did that "creeped up to 160 (as normal), then climbed fast to about 200", so I got the car moving to get airflow, and about 30 seconds later, the temp dropped to about 160, and stabilized. I drove and watched it for the next several days and realized it was dropping fast when the cold water in the radiator started flowing. Changed the thermostat (it had been in the car about a year) and never had another issue. Having a IR temp gun makes looking for any hot or cold areas. Can assist if there is blockages, like heater core/supply and return hoses, hot/clod cylinders etc.
Sounds like the thermostat is a little slow reacting. When I had the 160 stat in my car it would do that, too. I put a new one in (for other reasons) and it doesn't do that anymore.
