Tech question: When you're coasting downhill and the car is pushing the engine to high RPM, does it use gas? If so, does it use more than, like, idle gas?

 

Depends on the car, I guess. I think most cars use NO gas at all like that. If the throttle is completely closed & rpm is above 2000(?), it doesn't fire the injectors at all.

 

The Honda ECU does use a "fuel-cut-on-decel" strategy.

Engine RPM needs to be above 1100rpm and the TPS needs to indicate a closed throttle. Notice I say nothing about vehicle speed.

Ever wonder why you see sooooo many oscillating idle posts? This IS the reason. If there is an additional source of air entering the manifold (aka vacuum leak) this only serves to raise engine speed. Well, since the ECU sees the TPS is closed, it goes into the fuel cut strategy. So when the engine has warmed up (because a cold engine wants to idle at about 1500rpm anyway, which is why a defective ECT sensor always indicating that the engine is cold, or air bubbles in the coolant, will produce the same result), this vacuum leak allows too much air into the manifold, engine speed goes beyond 1100rpm, fuel injectors are cut off, engine speed falls below 1100rpm, fuel injectors cut back on....wash, rinse, repeat. The magnitude of how high the engine speed goes is mainly a factor of how large the vacuum leak is.

This strategy is also whats responsible for the jerking on acceleration (when you've already got the oscillating idle).