3 ms·

I've heard this before, and I understand how cutting the fuel supply to zero would save gas, but there are a couple of things that I don't understand: Wouldn't

I've heard this before, and I understand how cutting the fuel supply to zero would save gas, but there are a couple of things that I don't understand: 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? Why would built-in mileage computers, with direct access to actual fuel injection figures, compute gas mileage from some other measurement that's only theoretically related under some circumstances? Isn't that sort of going out of the way for worse results?

chris_j 16y ago
> 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.
- Lagged2Death 16y ago
  Perhaps a diesel engine is even more prone to the cooling effect, since (it is my understanding) diesels don't have throttles to limit the amount of air pumped through the engine on each rotation.