Dies when hot, possibly boiling gas, exhaust flap stuck

There is a specific lubricant and rust penetrant sold at the Mopar dealers for this purpose, and for other general stuck parts. It may help free the flapper. When I freed mine up on a '62, I just that an other penetrants for days. I eventually lightly tapped the shaft back and forth and back and forth (forwards and backwards on the car) a bit at a time while working in the Mopar penetrant and it slowly freed up and worked free enough so that I could start turning it some and eventually all freed up.