I drive an infiniti g20i. Some months back it over heated because of a bad radiator.i have changed the radiator gasket etc. Few months later i started hearing some sounds whenever i put on the Ac and reduces the spead of the car.its now worse that it will hardly move while the Ac is on and most times make the engine really hot. I was advised to get a new engine but a mechanic said its my crankshaft and that he can replace it to make it work perfectly. I am really confused .pls help