> Wouldn't this mean that coasting down a long hill in gear would cool the engine and in particular the catalytic converter, potentially causing some operating

> Wouldn't this mean that coasting down a long hill in gear would cool the engine and in particular the catalytic converter, potentially causing some operating hiccups and a big emissions spike when eventually reaching the bottom? I have observed precisely that. I have coasted down a hill in gear and have noticed the engine temperature (in my diesel engined car) begin to drop, which worried me a little at the time. This surprised me a little at the time because it happened in a matter of minutes, which I hadn't been expecting.