Quote Originally Posted by firebird77clone View Post
I don't know big blocks, but I suspect the thermostat.

Have you tried running it without? Or, better yet, if you have an old one, gut it and swap it, see what happens. Sometimes it is a good troubleshooting technique. It only costs a quart of coolant, gasket, and an old Tstat.
Just to cover the base, running without the t-stat is a great troubleshooting tool, but a terrible idea for normal operation. Slow to warm up to operating temperature (especially in frigid Maine), and in hot, humid areas will be prone to overheating because the residence time in the radiator is too low for heat transfer (coolant is traveling too fast to have time to cool). My opinion based on the experience of moving from MO to TX Gulf Coast 40+ years ago, and hammered into my head as the years passed. Like Tech pointed out, you've built in a bunch of potential headaches with your system. Don't make running without a t-stat another one.